'acrylonitrile-butadiene-styrene co-polymer' - a cross between plastic
and rubber with excellent engineering properties, used to make
underwater housings, cabinets for electronic equipment, solvent-weld
plumbing fittings, etc.. ABS is attacked by many industrial cleaning
agents, especially chlorinated solvents, ketones (e.g., MEK), and
furans (e.g., THF). Cleaning with agents other than mild detergent and
water is not recommended.
Deviation, Wandering away, The
non-convergence of rays of light to one focus.
current. An electrical current that changes over time, usually in a
sinusoidal fashion. Usually associated with mains electricity.
When Thomas Edison an
his co-workers developed electrical lighting into a commercially
practical technology, the original power distribution system used direct current
(DC). The problem with
DC is that it cannot easily be transformed from one voltage to another,
and so the generator has to be operated at something close to the
voltage which will be made available to the end user, i.e., a lowish
voltage with a correspondingly high current for a given amount of
transmitted power. Low voltage distribution systems unfortunately
suffer from the problem of resistance, or voltage drop, in the
transmission cables, and it was said that if a skyscraper had been
supplied with DC, the lights at the bottom of the building would have
glowed a lot brighter than the lights at the top. The problem was
solved by the adoption of a low-frequency AC system (largely
attributable to Nicola Tesla), ie., an electrical current which changes
from positive to negative and back again, sinusoidally, at a speed too
low to cause significant radiation loss but fast enough that no-one can
see the lights flickering. The beauty of AC power is that it can be
transmitted as high voltage, low current, and then changed to low
voltage, high current for the end user by means of a simple magnetic
device called a transformer (ie., two coils of wire on a common iron
core). In America, the finally adopted AC power-line frequency is 60
cycles per second (60Hz). In Europe, the power-line frequency is 50Hz.
The process of changing AC back into DC, for equipment which can only
run on DC (such as electronic circuitry and early 20th Century subway
trains) is called "rectification" (ie., 'putting-right').
a lens corrected for chromatic aberration
(colour fringing) at two wavelengths in the visible spectrum. Most
modern camera lenses are achromats.
a lens in which
achromatic correction is achieved by cementing together two lenses made
from different types of glass.
methyl methacrylate. A clear plastic with good optical properties and
very high transparency. Refractive index: nd
1.492. Trade names: Perspex, Plexiglass, Lucite. Acrylic is
attacked by chlorinated organic solvents. Stressed (i.e., shaped or
moulded) acrylic will shatter in contact with alcohols, so be very
careful about what you put on it if you want to clean it (mild
detergent and water is preferred). Wiping with 60-80 petrol (cigarette
lighter fluid) can be used to remove severe non-water-soluble
Automatic Exposure. A method of
linking the camera exposure settings to the ambient or reflected light
Automatic-Focusing. There are two basic types of
automatic focusing system; range-finding systems, and contrast or
image-sharpness sensing systems. Range-finding systems send out a
signal; usually an infrared beam (but sometimes ultrasonic), and see
how long it takes for the reflected signal from the subject to come
back. Such systems can be fooled by windows, and they don't work
underwater because the water absorbs or modifies the signal. Contrast
sensing systems determine image sharpness in much the same way as the
human eye, and are much more difficult to fool, but they can still be
fooled by repeating patterns, and they don't work if the subject lacks
detail. Range-finding systems are sometimes called 'active autofocus'
because they send out a signal, whereas contrast sensitive systems are
called 'passive autofocus' because they don't. This is one case where
it is usually better to be passive.
Ah, Ampere hours:
The capacity rating of
an electrical cell or battery. One point to watch when interpreting
ampere-hour ratings is that the capacity of a battery or cell varies
depending on the rate of discharge. The figure is normally specified at
the 10 hour rate. E.g., A 10Ah battery will deliver a current of 1 amp
for 10 hours. It will however, deliver a current of 10 amps for
somewhat less than 1 hour. I.e., the effective capacity is reduced at
high discharge rates (and also, not greatly increased at very low
discharge rates). In combination with the average battery terminal
voltage throughout the discharge cycle, the battery capacity is also a
measure of the available stored energy. E.g., a 6V 10Ah battery can
deliver 6 Watts (6 Volts x 1 Amp) for 10 hours, i.e., 6 Watts for 36000
seconds = 216000 Joules (216KJ). That is a lot of energy. If you
short-circuit a battery, nearly all of the stored energy will be
dissipated in the battery itself, which should explain why batteries
can pose a severe fire hazard if misused or mishandled. Some modern
batteries have energy densities close to that of Dynamite.
Al, Aluminium, Aluminum
: Humphrey Davy,
the discoverer of this chemical element, originally named it "A l u m i
u m". He then changed his mind and called it "A l u m i n u m", and
this naming convention is the one adopted in North America. The British
Chemical Society however, felt that element names should always end
with "-ium", and so changed the name to "A l u m i n i u m". The
Americans have the wishes of the discoverer on their side.
Amp, Ampere, A:
The unit of electrical
current, ie., quantity of electricity per unit time. Current flows
wires and conductors, whereas voltage (electrical
pressure-difference) appears across
generators, batteries and resistors.
Coverage (of a lens):
The angle between rays of light
entering a lens from the extreme corners of a scene.
Angle of Coverage (of a flash unit):
angle of the light-cone or pyramid emanating from the light source. The
quoted angle of coverage for a flash unit corresponds to the angle
between two rays, on opposite sides of the beam axis, chosen so that
the light intensity is half of that at the beam centre. I.e., it is
assumed that the useful field of illumination ends at the point where
the light intensity has fallen by 1 EV (1 stop) relative to the centre.
Circular light sources are given a single value for angle of coverage,
rectangular light sources are given two values (Horizontal and
(Anodization): An electrolytic
process for forming a stable film of aluminium (aluminum) oxide on the
surface of aluminium alloys. Aluminium will normally grow an oxide
layer on contact with air; but the layer formed in the anodisation
process is harder and less porous, giving improved corrosion and
abrasion resistance, and can be made to incorporate pigments as an
alternative to painting the finished product.
: Reflections that
occur at optical surfaces are a general source of lens flare (i.e.,
image artifacts due to bright light sources outside of the
field-of-view). Reflection is reduced by reducing the
abruptness of the change in refractive index at an optical surface.
This can be achieved by coating the surface with a very thin
layer of some material that has a refractive index intermediate
between the refractive indeces of the two media.
The variable diameter hole used
to control the amount of light passing through a lens. See f-stop
a lens corrected for chromatic
aberration (colour fringing) at three wavelengths in the visible
spectrum. Traditional apochromatic lenses using spherically-ground
glass elements are complicated and expensive. Apochromatic lenses can
also be produced by using aspherical moulded lens elements, but
moulding is generally an inferior process to grinding, and so, while a
lens with moulded elements may be strictly described as apochromatic,
that doesn't always mean that it will be superior to a traditional
achromat (and if the elements are pressed out of plastic it may well be
awful - caveat emptor). This is not to say that modern moulding methods
cannot produce results equal to grinding, they can, but high quality
optical mouldings are not to be expected in cheap lenses.
: Advanced Photographic System (now
Photographic film with integral magnetic recording strip for
data. 25.1 × 16.7mm image format.
: Term used for camera
sensors of format dimensions similar to those of APS.
: Anti-reflection (coating)
B, Bulb sync
In old-fashioned flash-bulb photography, the bulb must be ignited a few
milliseconds before the shutter is opened, to give it a chance to get
going. Alternatively, the shutter must be opened for a relatively long
time, to make sure that the bulb gives out its light while the shutter
is open. Nowadays, the camera 'B' position is used for time exposures,
it being a setting in which the shutter will remain open for as long as
the shutter button is held pressed.
: Screw-thread system for small
instruments proposed in the 19th
century by the
British Association for the Advancement of Science. BA screws are based
on metric dimensions with a constant pitch gradient. The pitch angle is
47.5°. Some BA sizes will fit with modern metric parts, e.g.
0BA fits M6×1, 13BA fits M1×0.25. Now largely
Light reflected from
suspended particles in the water, a problem in underwater flash
photography caused by having the flash too close to the camera lens. It
can manifest itself as anything from a slight fog, to a haze of bright
specks obscuring the picture. In extreme cases, particles close to the
camera shine so brightly that they cause lens flare, in which case some
of the specks will appear to have the same shape as the lens iris. The
problem is minimised by using an external flash, either mounted on a
long adjustable arm, or un-mounted and positioned by hand.
Metric unit of pressure (now deprecated in favour
of the Pascal, but considerably more intuitive). Standard
atmospheric pressure is 1.01325 bar. 1 bar = 100 000 Newtons / metre
squared (100 kilo Pascals). As you dive underwater, due to
the weight of the water above you; the pressure increases by 1 bar for
every 9.806 metres of depth (assuming fresh water). For sea
water (which is slightly denser than fresh water), we say, to a good
that the pressure increases by 1 bar for every 10m of depth.
|Bayer mosaic: The pattern
of red green and blue filters used to separate colour in a conventional
CCD or CMOS camera sensor. US Patent No. 3971065, 1976.
and resolution article for more
A measure of tonal
rendering capability in digital pictures. A good impression of
continuous tonal rendering can be created by having 256 possible levels
of brightness for each of the colours red, green, and blue. 256 levels
can be represented by an 8-bit (1 byte) binary number, so it takes 3 x
8 = 24 bits to create the impression of continuous tonal rendering. 24
bits corresponds to about 16.7 million possible unique values of hue
and brightness. It is better to start with considerably more than 8
bits per colour if any image adjustment is to be carried out, so that
there will be enough information left to give 8 bits per colour in the
A widely used high quality optical crown glass, relatively hard with
good scratch resistance.
= 1.5168 @ 587.6nm. More information
(named after the designers, Paul Neill and Carl Concelman). Coaxial
connector used for video, ethernet, and radio frequency signals.
[Rad Com (RSGB publ), October 1995, p67. QEX (ARRL
publ), May 1985, p2.]
(multi-pin) waterproof connector, manufactured by Bulgin, with an
ingress protection rating of IP68 1 bar (ie., submersible to a depth of
at least 10m, and in practice, a lot more).
The time for which a lamp will
operate after starting with a fully charged battery.
A wall or partition in maritime
connector which mounts on a panel or bulkhead.
The propensity of an object to
float (positive buoyancy) or sink (negative buoyancy) or to do neither
Sony always called its tape recorders 'Tapecorders', thereby avoiding a
pronunciation difficulty for native Japanese speakers. A combined
camera and video recorder therefore naturally became a 'Camcorder'.
Latin and Italian: a small room or
bedroom. English equivalent: Chamber. The word Camera came into the
English language as a shortened form of the phrase 'camera obscura', a
darkened room. The invention of the camera arises from the observation
that, if a house is completely shuttered, light coming through a chink
or keyhole will project an upside-down laterally-reversed image of what
lies outside on to the opposite wall. Artists developed the secret
practice of setting-up such a room (or tent, or box) and placing an
easel inside, so that they could sketch over the resulting image and
use it as the basis for a painting. A study by the artist David Hockney
(Secret Knowledge: Rediscovering the lost techniques of the Old
Masters, ISBN 0-500-23785-9) argues that the camera obscura, in
combination with various mirrors, lenses, and tracing screens, was used
by artists from about 1430 onwards - and that the works of many of the
Old Masters are effectively photomontages (the extraordinary skill of
the artist being that of rendering the optical image in paint). A
subsequent study of the use of perspective in Hockney's principle
example (Arnolfini & his Wife by Van Eyck) has argued that Van
Eyck did not use optical methods in setting out this work (see
Scientific American, Dec 2005, p52-59), although an alternative
interpretation is that the artist varied his optical setup depending on
which part of the picture he was rendering. The word 'photography'
means 'drawing with light', ie., dispensing with the artist, and using
light-sensitive chemicals instead. Because 'camera' means 'bedroom',
Italians use the terms 'fotocamera' and 'maccina fotografica'.
Closed-circuit Television, i.e., TV
not involving radio transmission or long distances. A somewhat
old-fashioned term that confuses the difference between video and TV.
Television means vision at long-distance, usually involving
transmission by radio, radio carrier in long-range cables, and
fibre-optics. The term 'CCTV' is therefore tautologous (short-range
long-range vision), and a CCTV camera is nowadays more sensibly
referred to as a 'video camera' or a 'surveillance camera'.
An electrical storage device.
In a flash unit, energy is stored in a high-voltage capacitor for rapid
discharge into the flash tube (See also inverter
Beware. of which to beware.
Charge Coupled Device. An integrated
circuit (micro-chip) consisting of a group of charge storage cells
(tiny capacitors) with the ability to pass charge from one to the next,
in a line, like a bucket-brigade (ie, like a group of fire-fighters
passing buckets from one to the next). Originally conceived in the mid
1960s as an analogue delay line for processing Radar images, someone
came up with the idea of exploiting the fact that the cells are
sensitive to light, and setting them out in an array, like the lines of
a TV picture. Thus was born the replacement for the fussy and
troublesome video camera tube. Most modern video cameras use a single
array with a colour mosaic filter to separate red, green and blue.
Early professional cameras split the light into red, green, and blue
optically, and used 3 CCD arrays, but as the number of pixels increased
(for HD and stills capture), physical alignment of the three pictures
became impossible, and single-CCD (mosaic filter) solutions had to be
(electrical): An electrochemical
energy source. A device with a particular voltage difference across its
terminals. Electrochemical cells are often connected in series to form
a 'battery of cells', commonly known as a 'battery'. Hence, for
example, a 12V battery may be constructed by connecting ten 1.2V cells
in series. A cell is composed of two half-cells, each comprisong an
electrode (often an electrically conducting rod, plate, film or
cannnister) and an electrolyte (a chemical solution). The two
half-cells may use the same electrolyte, in which case the two
electrodes are immersed in the same solution; or they may used
different electrolytes, in which case the two electrolytes are
separated by a semi-permeable membrane. The common Leclanché
cell (also known as the zinc-manganese cell), has a positive electrode
consisting of a porous container filled with manganese dioxide powder
with a carbon rod inserted into it for connection, a zinc can for the
negative electrode, and a solution of ammonium chloride for the
: When two dissimilar
metals in contact are immersed in water containing dissolved salts (an
electrolyte), a short-circuited electrochemical cell is created. The
action of the cell is to etch or consume one of the metals thereby
producing electrical energy which is immediately wasted as heat. One
way to stop the corrosion is to reverse-bias the corrosion cell, i.e.,
apply a voltage in opposition to the voltage generated so that no
current flows. This is done by creating an additional corrosion cell
using a metal which is more readily consumed than the metal to be
protected. Aluminium, for example, can be protected by attaching a
block of magnesium to it, this attachment being called a 'sacrificial
: Screw-in lens mount used on some
ciné and video cameras. 1" diameter thread with 1mm pitch.
Cyan, Magenta, Yellow. The
'subtractive' primary colours. Colour-space representation used in
conventional print photography.
Cyan, Magenta, Yellow, Black. The
colour-space representation used by ink-jet and other printers. The
letter 'K' is used for black because 'B' already stands for blue. The
extra black ink is used to overcome the density limitations of the
other inks, i.e., C+M+Y usually comes out dark brown rather than pure
black. Also, it is a waste of money to print black and white documents
and text using exotic coloured inks.
: Colour is what passes for
spectroscopic analysis in the rather limited human (i.e., primate)
visual system. A pure monochromatic light source will produce an
impression of colour that depends on the wavelength of the light
falling on the retina. The eye and the brain work together to analyse
the input according to the relative levels of stimulation received by
three types of cone cell, which have sensitivity peaks at wavelengths
of 560 nm (red), 530 nm (green), and 424 nm (blue) (nm = nano metres).
crudeness of this analysis makes colour photography (as we know it)
possible, because we can fool the eye into seeing almost any colour
simply by superimposing three light sources (R, G and B) and adjusting
their relative intensities. Photographic (synthesised) colour is an
illusion specifically tailored to (and exploiting the limitations of)
the primate visual system; so don't expect non-primate animal species
(even other mammals) to make sense of photographs and TV pictures. For
more information on the evolution of colour vision in vertebrates and
the differences between species, see "What Birds See", by Timothy H
Goldsmith, Scientific American July 2006. p50 - 57.
: The set of colours which can
be represented by a particular imaging system. There is considerable
latitude of choice in selecting the exact wavelengths used to stimulate
the red, green, and blue receptors (cone cells) in the eye. Different
choices, and imperfections in the spectral purity of the light sources
or dyes used, affect the colour representation capability (colour
space) of the system. Synthetic (RGB or CMY) colour systems moreover,
cannot properly recreate colours which correspond to the violet (450 -
390nm) part of the spectrum (magenta = red + blue is the best we can
do), which is why photographs of bluebells always come out wrong. When
editing images, it is best to use a working colour space which
corresponds reasonably closely to that of the intended display device.
The temperature of an
equivalent hot black body. A theoretical black body is a perfect
absorber of light when cold, and hence a perfect emitter of light when
hot (i.e., when glowing). The black body is equivalent to the light
source in question in the sense that it produces the same relative
amounts of red, green, and blue light. Colour temperature is measured
K (degrees above absolute zero).
If an object is heated in a
furnace, or by passing an electric current through it, it glows with a
particular colour (red, orange, yellow, white, or bluish white)
depending on the temperature. If the emitted light is free from spikes
or gaps in its spectrum at particular wavelengths (ie., colours), then
it is known as 'black body radiation'. Tungsten light-bulbs produce
good black-body radiation. Electronic and gas-discharge light sources
(e.g., flash tubes) do not produce perfect black-body radiation, but if
there is enough red, green and blue light in the mixture, the source
can often be correlated to a black body at a particular temperature.
The correct term to express the colour rendering quality of a
particular light source is therefore "correlated colour
". Standard photographic daylight is assumed
to have a colour temperature of about 5500K. Xenon flash has a
correlated colour temperature of about 6000K, which is fine for most
purposes, but sometimes a little too blue. Underwater flash makers
sometimes use a yellow reflector or filter, to reduce the flash colour
temperature to around 4500K, to compensate for the blueness of the
water. Domestic tungsten light-bulbs have a colour temperature of about
2950K, which is very orange. Quartz-halogen lamps have a colour
temperature of 3200 - 3400K, which is yellowish, but electronic cameras
can compensate for this by adjusting the relative sensitivities of the
red, green, and blue channels (white balance). Humans do not usually
notice the effective temperature of a light source except by
comparison, because our eyes adjust automatically (look at the colour
of a tungsten light-bulb while there is sunlight streaming through the
windows - the electric light seems orange or yellow in the daytime, but
perfectly white at night).
The colour of light
that, when added to a given colour in the correct proportion, produces
white (or grey, i.e., an illumination without a colour cast).
gives white. Green
gives white. Blue
gives white. Hence Red and Cyan are a pair of complementary colours, so
are green and magenta, so are blue and yellow.
(of data): The removal of
redundant or repetitive information from a data set (e.g., a file) in
such a way that the original data can be reconstructed.
compression that can be
reversed without loss of information (e.g., LZW).
compression that does not
permit exact reconstruction of the original data (e.g., JPEG).
A lens in which the
principal optical surface curves into the body of the glass, ie., the
glass has a a hollow or dent.
A lens in which the principal
optical surface bulges outwards.
: Cathode Ray Tube. Term used for the
large glass vacuum tubes that were used in television or video displays
oscilloscopes (but have now been superseded by flat panel displays).
The term was coined in the
19th century, before anyone knew that the mysterious 'cathode rays'
that would cause a zinc sulphide screen to glow were actually the
sub-atomic particles we now call electrons. It became
change the archaic name, because the Americans adopted the term
'electron tube' to refer to the device that the British scientist
Ambrose Fleming preferred to call a 'valve' (because it only conducts
electricity in one direction). Fleming's valve was one of Mr Edison's
light bulbs with an extra plate inside it to capture the electrons
emitted by the hot filament. A CRT is a type of electronic valve (or
electron tube) in which the electrons are focused into a beam and fired
a phosphorescent screen. In a video display, magnetic fields produced
by coils of wire fitted to the neck of the tube steer the electron beam
rapidly so that it can be used to trace a series of horizontal lines on
the screen (a raster
'Chrominance, Video, Blanking, and
Synchronisation', ie., a composite video signal.
Digital 8: Digital video recording system using 8mm tape cassette.
Direct current (as opposed to alternating current
); an electrical
current that does not alternate or change significantly with time (at
least over a moderate interval), such as might flow when a light bulb
is connected to a battery. Never say "DC current" (direct current
current), just say "DC".
An electronic device that
converts from one constant voltage to another (usually higher)
constant voltage, e.g., 6V in, 330V out, as used in typical small flash
units. A DC-DC voltage up-converter may also be called an inverter
Acetal Homopolymer. Engineering
plastic with sufficient hardness and dimensional stability for the
manufacture of precision components (e.g., screw-threads).
Depth of field:
No lens can produce a
perfectly sharp image. This means that there will be a range of focus
settings over which the lens will give no significant improvement over
an exact setting. Conversely, for a given distance setting, there will
be a range of lens to subject distances over which focusing will be
adequate or not-improvable. This range is called the depth
. The depth of field of a lens improves as the
aperture of the lens is made smaller, although maximum depth of field
is not the same as maximum optical resolution. The trick, in
photography, is to choose the aperture so that the depth of field
embraces the whole depth of the subject from the furthest to the
nearest point. Good lenses are provided with depth of field markings to
assist in this matter, and good SLR cameras have a 'depth-of-field
preview' button, which closes the lens down to the aperture which will
be used when the picture is taken.
A device that increases the
effective area of a light-source, i.e., makes it less point-like. A
piece of opal or translucent plastic that can be fitted to the front
of a light source. A diffuser helps to even-out the distribution of
light intensity in the field of illumination, and reduces the problem
from the subject.
: In-camera image cropping.
A feature beloved of marketing departments for the purpose of
making the camera zoom-range specification seem better than it
: A unit used to express the power
of magnifying glasses. Screw-in magnifying lenses, which can be fitted
to the front of a camera lens, are sometimes referred to loosely as
The power of a lens
in diopters is the reciprocal of its focal length in metres,
(Diopter / 4) +1
|Maximum Focus Distance
1m / Diopter
The neat thing about diopters is that if you stack several
lenses, the new magnifying power can be obtained simply by adding
together the diopters of the individual lenses; i.e.,
D = D1 + D2 + . . . . . . . etc.
This formula holds good only if the lenses are close together, i.e.,
well inside each other's focal distance. Thus, if you stack a 1 diopter
lens with a 2 diopter lens, you get a 3 diopter lens; but that doesn't
mean it's a good idea. Stacking lenses like this is a useful practice
while you're trying to determine the magnification power needed for a
particular job; but if you want best corner to corner image sharpness,
you should then obtain a single lens of the right power.
Lenses that are
overall convex (bulging) have positive diopters. Lenses that are
overall concave (indented) have negative diopters. The low-cost
close-up lenses designed to screw into the camera filter ring are
usually convex on one side and concave on the other, but the convex
part is more curved than the concave part, so overall, it's a
magnifying glass. This concave-covex structure is called a `meniscus'
lens, and is used because it gives the best minimisation of chromatic
aberration for a lens constructed from a single piece of glass.
Needless to say, adding such a lens to your camera causes a reduction
in optical resolution. Far superior results can be had, at a price, by
using two-element achromatic close-up lenses, such as those
manufactured by Nikon and Canon.
||Meniscus lenses for screwing into the camera-lens
filter thread. The lens holder has a male thread at the back and a
female thread at the front, to allow stacking and the use of additional
The separation of white light
into its component colours. Dispersion in optical media (such as glass)
occurs when the refractive index varies with wavelength. It is
generally undesirable in lenses, since it leads to chromatic aberration.
Digital Noise Reduction. Clean-up
process used when transferring analog video signals into the digital
Lens port constructed as part
of a sphere of uniform thickness. The dome port corrects for the
magnifying effect that occurs at an air-water boundary, and thus
preserves the image geometry and angle of coverage. Contrary to popular
myth, the dome port does not correct for all optical aberrations at the
boundary, and actually introduces some. Introduced aberrations are
reduced as the radius of the dome increases, but the problems of
excessive buoyancy and delicacy of the outer optical surface impose
practical limits on dome size.
: Sea & Sea
proprietary underwater connector used on sync. cables and flash units.
It is called 'Duo' because the electrical specification supports both
the Motormarine II and Nikonos V flash interface protocols.
Pattern printed on the side of
a 35mm film cassette that is used by the camera to sense the required
ISO/ASA film-speed setting. The film speed is sensed by means of
electrical contacts pressing against the canister, so dirt can cause
mis-sensing. DX-coding was meant to simplify photography; but if no
over-ride was provided, it could be a pain for advanced photographers
wanted to deviate from the film manufacturer's recommendations.
Radio waves, infrared, visible light, ultraviolet, X-rays and gamma
rays are all manifestations of the same thing: electromagnetic
radiation. The difference between the various types is a consequence of
the difference in frequency (or wavelength). The theory was worked out
in the 19th
century by James Clark-Maxwell; who
showed that electricity
and magnetism are related and that light consists of electric and
magnetic fields linked together as they travel through space. Maxwell's
work set the stage for the development of radio and countless other
technologies that we nowadays take for granted. The relationship
between frequency and wavelength is:
c = f λ
where c is the
speed of light (299 792 458 metres per second), f is the frequency, and
λ (the lower case Greek letter lambda) is the wavelength,
ie., the distance the wave travels as it undergoes one complete cycle.
The middle of the medium-wave broadcast band is about 1MHz (one million
cycles per second) and the corresponding wavelength is 300 metres. The
wavelength at the middle of the visible spectrum is about 500nm
(nanometres = thousand millionths of a metre), and the corresponding
6 × 1014
Hz, ie., 600 000
000 000 000 Hz (600 THz or 600 Tera Hertz).
Radio signals are normally
characterised by their frequency, but light is normally characterised
by its wavelength because the numbers for frequency are messy to write
down without using scientific notation.
radiation is also sometimes split into two classes: ionising radiation,
ie., radiation that has sufficient energy to knock electrons out of
atoms and molecules, and non-ionising radiation, which hasn't. Radio
and infrared are classed as non-ionising radiations (although some
things can be ionised by infrared), whereas visible light, UV, X-rays
and gamma rays are definitely ionising radiations. Our body senses are
divided according to this distinction; in that we sense infrared
through the skin by its heating effect, whereas we sense visible light
through our eyes by its ability to split molecules of light-sensitive
pigment. Insects can also see in the UV part of the spectrum, whereas
to the human body, anything of shorter wavelength than visible light is
Electro-Oceanic: A type of wet-connector
with two contacts, in
appearance like a rubber jack-plug.
Errors and omissions
excepted. Legally, this means that the information given cannot form
part of a contract, i.e., it is not binding on the seller because it
may require corrections.
: Exposure value. A change in exposure
of 1EV corresponds to a halving or doubling of the intensity of light
falling on a film or sensor, i.e., a 1EV change is equivalent to a
change of 1 f-stop, or a halving or doubling of shutter speed or ISO
Pronounced "ay-va marine".
German manufacturer of flexible underwater camera housings.
Such housings are known in the film and broadcast industries
: Electronic Viewfinder,
Exchangeable Image File. An extension to the JPEG file format to
include information provided by a digital camera: time, date, firmware
version, focal length, aperture, shutter speed, exposure value, etc.
: The product: light intensity
: the practice of
taking a series of photographs, usually at 1EV or ½EV
either side of the expected correct exposure, in order to obtain a
range of images with slightly different exposures from which the best
may be chosen. Exposure bracketing is of particular value when using a
recording medium with limited exposure latitude, such as slide film, or
digital cameras that only give 24 bit output.
A lens focuses at infinity when the distance from the exit pupil of the
lens to the film or sensor (the focal plane) is equal to the focal
length. As the lens is moved away from the focal plane, closer objects
come into focus, but the amount of extension permitted by the focus
barrel (the thing you turn) is limited at some point by a decision made
by the lens manufacturer. To make a lens focus closer than the designer
originally intended, an extension tube can be fitted between the lens
and the camera. Extension tubes for system (SLR and electronic
viewfinder) cameras can
have have control linkages or electrical connections passing through
them to retain any automatic functionality of the lens. Manual
extension tubes are simply tubes, with a male lens mount on one end and
a female mount on the other. Different length tubes give different
ultimate magnifications with a particular lens. Note that unless the
lens is designed for the purpose, it will be taken out of its optimum
focusing range when a tube or bellows extension is used and will
therefore not give its maximum possible resolution.
Generic term or collective noun for nuts, bolts, screws, washers, and
other parts or devices used in attaching one object to another.
: Iron (ferrum)
Field of View, FOV
- of a lens: same as angle of coverage
Any device that modifies a signal
is a filter. Therefore, it is legitimate (but not good usage) to
describe a close-up lens as a filter, the real reason for doing so
being that close-up lenses screw into the camera filter-ring. In
photography, the term 'filter' is normally reserved for a device
(usually a sheet of glass or plastic) that attenuates (reduces) all or
part of the light passing through it. A Neutral-Density (ND) filter
causes an equal reduction across the whole visible spectrum and is used
when the light is too bright for the camera equipment, or to modify the
required iris setting as a way of controlling depth-of-field. Most
other filters modify the colour or the light in some way, or remove
troublesome invisible components. A UV filter removes ultraviolet
light, to prevent haze, but leaves the visible spectrum virtually
unchanged. An underwater colour-correction (UWCC
filter has an orange
or magenta colour to compensate for the cyan-blue or green colour cast
introduced by photographing through water; an there are a host of other
colour-modifying filters for different applications.
: Except for the products of
awkward manufacturers, camera filter screw threads are international
standard (ISO) metric (M) threads with a pitch distance of 0.75 mm. If
you measure the inside diameter of a female filter thread (using
engineer's calipers), adding the pitch distance to the result gives the
nominal thread diameter plus about a 10th
mm. E.g., a camera
filter thread measures 57.35 mm. Adding the pitch distance gives 58.1
The filter thread size is fully specified as M58×0.75, but
usually abbreviated to M58.
Wide angle lenses can be designed
in two ways: They can be corrected to make all straight lines appear
straight (rectilinear correction); but this type of correction
introduces perspective distortion, ie., assuming a flat object, it
exacerbates the fact that points at the edge of the field are further
away than points in the middle. The alternative is to avoid rectilinear
correction, in which case perspective distortion is minimised; but
rectangles appear to bulge in the shape of a barrel, and the only lines
that remain straight are those that pass through the exact centre of
the field of view. The latter type of lens is called a 'fisheye' lens.
Fish-eye distortion of rectangular test card
with vanishing point
placed at the centre of the image
A major problem in lens design is
that of minimising reflections from the surfaces of the lens elements.
Such reflections result in multiple images, and in light from bright
objects outside the field of view arriving at the film or sensor. The
various optical effects that can occur due to internal lens
reflections are known collectively as 'flare'. Reflections occur at
boundaries where there is a sharp change of refractive index (e.g., on
going from air into glass) - the sharper the boundary the stronger the
reflection. Modern lenses have anti-reflection coatings, which make the
change of refractive index more gradual and so reduce (but do not
eliminate) flare. All compound lenses will exhibit some flare when
shooting into the sun. Strong reflections from scene highlights will
also cause visible flare. Some lenses can be fitted with an external
'anti-flare hood', which is designed to cut off all light-rays that do
not originate from objects within the field of view.
Focal length, f:
The distance at which a
lens brings light from infinity to a point. The focal length of a
simple symmetrical lens, is the distance between the image plane (the
surface on which the image is formed) and the middle of the lens, when
light from infinity is brought to a focus. You can estimate the focal
length of a simple lens by projecting an image of the sun onto a
heat-resistant surface, and measuring the distance from the middle of
the lens to the image (do not look at the sun through a lens, do not
focus an image of the sun onto your skin). Strictly, the place to
measure from is the exit-pupil of the lens, which is where the iris
(aperture) appears to be when you look into the back of the lens. Most
camera lenses are not symmetrical however, i.e., the actual focal
length is different if you turn the lens around, in which case, the
figure quoted is the focal length of an equivalent symmetrical lens.
Many camera lenses also are of the retrofocus type, i.e., they have
additional optics at the rear to allow them to sit close to the film
plane, and the simple method for estimating focal length given above
will produce misleading results.
talk of focal length in the context of a given camera format (usually
36 × 24mm), in which case the term serves a shorthand for the
associated angle of coverage. Note however that the commonly understood
relationship between focal length and angle of coverage breaks down
when you change the optical medium; i.e., in the absence of a
correcting element (such as a dome port) lenses of a particular
specified focal length are not as wide-angle underwater as they are in
The 'format' of a film or camera
is the dimensions of the image area (or frame). The 35mm (full-frame)
film format used by stills cameras is 36 x 24mm. The 35mm (half-frame)
format, used by motion-picture cameras is 24 x 18mm. The 2¼"
square (medium) format is 57.15 x 57.15mm (not 60 x 60mm as is
Digital cameras use a
wide variety of formats, making it extremely difficult to relate the
focal lengths of the lenses used to the actual angles of coverage
achieved. This problem is usually solved by giving a focal
, i.e., a number which when multiplied
by the actual focal length, gives the equivalent focal length for the
35mm film format.
For a list of format sizes and focal
length multipliers, and the implications of format choice with regard
to image quality, see the Image formats
: An "open-standard" image
format and common lens-mount specification for high-quality digital
cameras, intended to make lenses from different camera manufacturers
interchangeable (see http://www.four-thirds.org/en/
It is supported by Olympus, Kodak, Fujifilm, Panasonic, Sanyo, Sigma,
and others. The specification dictates a senor diagonal of 21.63mm
(half that of the 35mm format, giving a focal length multiplier of 2).
The original Four Thirds system was intended for SLR cameras. A variant
"Micro Four Thirds" has a reduced lens to focal-plane distance
specification and is designed for for mirrorless live-preview
(electronic viewfinder, interchangeanle lens)
cameras. Four Thirds lenses can be used on Micro Four Thirds cameras by
means of an adapter (an extension tube with electrical contacts).
: Field of view. Usually synonymous
with Angle of Coverage
The illuminating power of a lens is a function of the focal length of
the lens and the diameter of the hole (aperture, iris) through which
the light is allowed to pass. The amount of light falling on a film or
electronic sensor can be adjusted by changing the diameter of the
aperture; but instead of recording the actual diameter, we can treat
all lenses equivalently if we record the aperture diameter as a
function of the focal length. Thus we record apertures as f/n (focal
length divided by a number n). eg., a 50mm lens at f/2 has an aperture
of 25mm, an 80mm lens at f/2 has an aperture of 40mm, but both have the
same illuminating power. A standard series of apertures has evolved
such that the illumination doubles or halves with each click-stop
(f-stop). If you double the diameter (or radius) of a circle, you
quadruple the area (area = πr²), and it is the area
that determines the level of illumination. Thus, to get from one
f-stop to the next, the number n is not doubled, but multiplied by the
square root of 2 ( √2 = 1.414). The standard f-stop series
thus comes out as: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16,
f/22, f/32, etc., ie, the number doubles for every 2 steps in the
series. The smaller the number n, the larger the aperture. The aperture
designation "f/22" incidentally is an ancient mistake. A more exactly
calculated aperture value is f/22.6 which, when rounded to the nearest
number, should really be called "f/23".
'Graphical Interchange Format'. A compressed image file format best
suited to diagrams and images with a restricted colour range. In GIF
compression, the colour range of the image is first reduced to 256
colours or less, then the remaining information is further reduced in
size by a scheme known as LZW (Lempel-Ziv-Welch) compression. LZW
compression is lossless, which means that you can edit and re-save the
image without degrading it, but the initial colour-space compression
makes the format unsuitable for high-quality colour photographs. Note
however, that 256 levels in a black and white image can be indexed
as colours, which means that the GIF format is lossless for 8 bit
continuous-tone monochrome images. The LZW algorithm was
patented by Unisys. The US patent expired in
2003, but international patents required the payment of licence fees in
other countries for some years after that. That situation led
to the development of the alternative open-source PNG format.
: In underwater engineering parlance:
an joint or interface that prevents water ingress; e.g., control
gland, cable gland, etc. A sealing structure that permits passage
through a bulkhead (vessel wall), usually a combination of machined
metal or plastic parts and O-rings.
: Gate Turn-Off Thyristor. A power
switching device used to switch-off (quench
) the current in a flash tube
in order to achieve automatic or TTL flash exposure control. Now
largely replaced by the IGBT
A number used to represent
the illuminating power of a photographic flash. When electronic flash
is used, the camera shutter opens (i.e., the exposure begins), the
flash fires, and the shutter closes; but because the flash duration is
very short (and assuming relatively little ambient light), the
'shutter' does not control the amount of light falling on the sensor or
film. Consequently, the level of exposure due to the flash can only be
controlled by the lens aperture (f-stop
setting. In metric countries, the flash guide number is the aperture
setting required for ISO 100 sensitivity when the flash to subject
1m (in North America, the guide number is the 100ASA aperture setting
for a distance of 1 foot. Divide the American guide number by 3.28 to
get it in metres). Using the Guide number G, the required aperture
setting for any ISO speed and distance (in air) is obtained thus:
This equation is used to produce the guide table that is sometimes
attached to the flash.
Hole Accumulation Diode. Diode structure used in some types of CCD
|Hand-wheel (nut ,
bolt ): A
threaded nut or bolt that can be tightened
by hand, usually in the form of a knob with a gripable (knurled)
surface, or with protrusions (wings).
Quasi-English term (malapropism)
used in Germany to mean 'Mobile Telephone', and used in other countries
to mean 'hand-held'. In native English idiom, handy simply means
A method for recording
analog video signals onto magnetic tape.
The problem in recording
a video signal is that it has enormous
bandwidth in comparison to an audio signal. This means that the tape
must move at a very high speed relative to the recording head. Tape in
an audio cassette moves at 47.625 mm/sec (1+7/8 ips). To record analog
video, the tape would need to move at about 5 - 10 m/sec (11 - 22 mph)
depending on the required picture quality, which is
solution is to mount recording heads on a drum, which is set at a
slight angle to the path of the tape and spins rapidly while the tape
moves by slowly.. The result is that the recording heads execute a
helical path relative to the movement of the tape, and thereby write
the video information at high speed as a series of diagonal stripes.
The drum rotation is synchronised with the picture, so that
the changover from one stripe to the next occurs in a non-displayed
part of the video waveform (the field blanking interval). All
analog video-tape recorders from the late 1950s onwards used helical
scanning, and the technique was also used to write CD-quality audio on
to tape in DAT (digital audio
High-band Video 8.
(Lamp): High Intensity Discharge Lamp.
Analog video signals are written onto tape as
a frequency-amplitude modulated (FAM) radio-frequency signal. The
frequency modulated (FM) part represents the luminance (brightness)
information, and the amplitude modulated (AM) part represents the
chrominance (colour) information. Early systems used a relatively
low-frequency range for the FM, but as the sizes of the magnetic
particles in the tape became smaller with improvements in the
technology, higher frequency recordings became possible, with
consequent improvements in the available video bandwidth. This led VTR
manufacturers to release high-band versions of their recorders (e.g.,
S-VHS and Hi-8). The high-band machines can record and play the earlier
tapes in the low-band format, but the old low-band machines cannot play
the high-band tapes, and cannot record or erase them properly because
higher magnetic field-strengths are required. A high-band machine can
however, make a low-band recording on a high-band tape for playback in
a low-band machine.
photography; a graph of the number of pixels occupying each possible
brightness level. Experienced picture editors learn to look at the
image histogram and assess the 'health' of the image in terms of what
it will look like when printed. It there are serious gaps in the
histogram (brightness levels not used or 'not populated') the picture
will not be able to give the impression of continuous tonal gradation,
and the print will appear 'posterised', ie., made out of discrete blobs
of colour with obvious boundaries between them.
|Hot Shoe: A camera
accessory shoe with electrical connections for a flash unit. Different
manufacturers use different connector pad layouts and electrical
signals. A standard accessory shoe without any electrical connections
is sometimes also erroneously called a hot-shoe, so that, for example,
Sea & Sea made a flash arm to fit on the accessory shoe and
called it a "hot-shoe mini-arm".
particularly in the Maldives, to refer to
the underwater surroundings of a small coral island diving resort.
Also, through adoption as diver's idiom, used
refer to the reef local to any diving resort. It is almost
certainly a homophone of
the German 'Zuhause riff' (home reef).
That part of an
underwater camera or other equipment in which the waterproof seals are
located. A casing or box with waterproof seals, designed to contain a
camera or other equipment in such a way that it can be used underwater.
microphone. Because air is an easily compressible medium, but water is
not; sound waves (pressure waves) travelling in water involve small
displacements of the water molecules, whereas sound waves travelling in
air involve relatively large displacements of the air molecules.
Consequently, if you put an ordinary microphone in a box, the sound
waves in the water won't move the walls of the box very much, won't
compress the air in the box very much, and so won't move the diaphragm
of the microphone very much. The solution is to have a specially
designed microphone in which the diaphragm is actually in contact with
the water and is sensitive to very small displacements.
Bulkhead mounting hydrophone.
(of a lens): The
distance setting that gives the greatest possible range of distances
at which objects appear to be in focus. Good lenses have means of
indicating the depth of field
for a given aperture setting. The hyperfocal setting for a given
aperture is obtained by placing the most distant limit of the depth of
field range on ∞. The corresponding 'hyperfocal distance' is
the actual distance setting when the hyperfocal criterion has been met.
Note that wide-angle lenses have an extremely large depth of field, and
therefore, unless being used for extreme close-up work, are best set to
the hyperfocal distance and left alone (refocusing is only necessary if
the aperture is changed).
Hz, Hertz, c/s:
Hertz is the unit of
frequency, and is synonymous with 'cycles per second' (c/s). The unit
was established in commemoration of the work of Heinrich Hertz, whose
scientific experiments in the late 19th century were the basis of radio
Ikelite connector system.
A 2-pin wet connector used with
early Ikelite manual flash units.
A 4-pin O-ring sealed connector used
with early Ikelite automatic flash units.
A 5-pin O-ring sealed connector used
with current Ikelite TTL flash units and video lights.
(3rd pin is for reversal prevention only)
: Insulation Displacement Connector.
Insulated Gate Bipolar Transistor. A general purpose power switching
device, which finds application in photography as the series control
(tube switch-off) element in automatic and TTL flash units. The IGBT
control circuit is nowadays preferred over the old Vivitar GTO (Gate
Turn Off) Thyristor circuit, because it can reset ready to fire in
about 30 ms (as opposed to 100 ms for the GTO), and so gives better
performance in systems that use pre-flashes for exposure evaluation
(i.e., with digital cameras).
The founder of the company was
Ike Brigham, and the first product was an underwater light, hence
Digital video interface (connection between camera and editing
equipment, monitor, computer, etc.) specified in document IEEE 1394.
Also known as 'FireWire'. Data transfer speed up to 400 Mbit/s.
: The Metric and Inch measurement
systems were finally made compatible in the 1960s by a small adjustment
to make 1" = 25.4 mm exactly. This made it possible for any
to manufacture exactly according to either system by the inclusion of a
127-tooth gear in the screw-cutting gearbox. Note that all
American / Imperial measurements (inch, ft, lb, etc.) are now based on
: Changing the number of
samples in a data set (e.g., pixels in an image) by estimating the
values for intermediate points for which no data exists.
A device that produces a high voltage output from a low voltage source
(sometimes also called a DC to DC converter). In portable photographic
flash units, an inverter is used to charge a high-voltage capacitor
from a battery, which is why the internal circuitry of a flash gun is
dangerous even though the batteries can be handled with impunity. The
inverter in a flash unit is an oscillator, which usually runs at a
frequency in the audible range and so makes a whining noise.
IP rating: Ingress
rating to BS EN 60529: 1992
Beneath red. That part of
the electromagnetic spectrum (light) that lies just below red in
frequency (ie., of longer wavelength than red).
Synonymous with 'aperture', but now
somewhat old-fashioned; the variable diameter hole used to control the
amount of light passing through a lens. See f-stop
: Image Stabilisation.
: Industrial, Scientific, and Medical.
: International Standards Organisation.
The basic standard for recording
computer data files onto Compact Disc. ISO 9660 allowed only for upper
case 8.3 type filenames, using the characters A to Z, _ (underscore)
and - (minus or hyphen). Because of the limitations of ISO 9660,
various extensions to the standard were developed to permit the use of
filenames and more characters, but these extensions are not always
across the various computer platforms (Microsoft, Linux, UNIX,
Macintosh). Consequently, if you want to create a disc that can be
read by all current computer systems, although you can use long
filenames and lower case, it is important to avoid spaces and
non-alphabetic symbols other than - and _.
ISO film speed:
The method of film speed
(sensitivity) classification developed for the American Standards
Association (ASA) and finally adopted by the International Standards
Organisation (ISO). The scale is such, that if the ISO speed number of
the film is doubled, the sensitivity of the film is doubled, eg., if
for a given lighting situation, you change from ISO 100 film to ISO 200
film, you must either close down the lens aperture by one stop, or
double the shutter speed.
The unit of energy. 1 Joule = 1 watt second.
: Japanese Industry Standard.
Joint Picture-Expert's Group. A
full-colour image file format with variable compression. JPEG
compression is always lossy (there is no such thing as a lossless
JPEG), and so an image should only be converted to this format when all
of the editing has been finished. Lightly compressed JPEG images are
only distinguishable from the uncompressed original by careful
comparison under high magnification. Heavy compression (all too
prevalent on the web) produces very small files, but the quality is
awful. It is better to keep picture sizes small than to display large
images with obvious compression artifacts. See also EXIF
The unit of absolute temperature, ie., degrees
above absolute zero. Named after Lord Kelvin, who was the first to show
that there is a degree of coldness below which it is not possible to
go. 0K = -273.16° C. When a temperature is expressed in Kelvin,
the ° symbol is not used, ie., eg., 0°C = 273.16 K. See
also Colour Temperature
: Potassium (kalium).
A short piece of rope or line used as a handle or tied-on to some other
object. A wrist-lanyard is a loop of thin rope with a sliding toggle,
which may be tightened around the wrist to secure the object (usually a
torch or a camera) attached to it. When SCUBA diving, it is better to
secure a camera by means of a wrist-lanyard than by using a neck-strap,
since the mouthpiece must be removed in order to take a neck-strap on
Liquid Crystal Display.
Light Emitting Diode.
An extremely tough polycarbonate plastic, usually transparent, used to
make underwater housings. 'Lexan' is a trademark of the General
Electric corp. See polycarbonate
: Lempel, Ziv, Welch. A fast lossless
data compression algorithm patented by Unisys (see also GIF and TIFF).
Ref: "A Technique for High Performance Data Compression", Terry A.
Welch, IEEE Computer, Vol 17, No 6, 1984, p8-19.
Prefix used to indicate a screw-thread conforming to the ISO
(International Standards Organisation) Metric system (60° pitch
angle). A metric thread specification includes a nominal diameter in mm
and a pitch distance (i.e., distance in mm between adjacent turns of
the thread), e.g., M6×1 = 6 mm diameter, 1 mm pitch;
M8×1.25 = 8 mm diameter, 1.25 mm pitch. The pitch distance is
often included because each diameter is available with a range of
pitches from coarse to fine; but if the pitch distance is omitted, it
is usually assumed that the coarsest thread is implied: e.g., the
thread of a Nikonos connector is M14×1, specifying it as M14
will not do, because the coarsest 14 mm thread is M14×2.
Camera filter threads are an exception to the 'always state the pitch
unless it's the coarsest one' rule however because, apart from the
deliberate awkwardness of some manufacturers, the filter thread pitch
is 0.75 mm. To find the tapping hole size for a metric thread, simply
subtract the pitch from the nominal diameter; e.g., for an
M6×1 socket, the tapping drill is 5.0 mm. Note when measuring
threads, that a screw is always slightly smaller than the nominal
diameter, and a socket (threaded hole) is slightly larger. This
difference (usually a few 10th
s of a mm) is
necessary to ensure that
the parts will fit together and turn smoothly.
Larger than life. Strictly, a macro
photograph is one where the image recorded on the film is larger than
the subject. The term is used loosely however, to describe an image
that can easily be printed or projected `larger than life'. Cameras
are usually adapted for macro photography by adding a lens extension
tube or a supplementary lens, or in the case of an SLR, by fitting a
lens that can focus at very short distances.
An extreme close-up lens. This
may be a complete camera lens, or a lens attachment that screws into
the filter ring or bayonet mount of the main camera lens. A macro lens
is usually associated with a ratio such as 1:1 or 1:2 etc. The first
number is the relative size of the image on the film, the second number
is the relative size of the subject, so read a:b as "a on the film, b
in real life". SLR macro lenses are usually focusable from infinity
down to 1:1 (life-size on the film), or 1:2 (half life-size on the
film). Nikon refers to its SLR macro lenses as `micro' lenses, e.g.,
`60mm AF Micro-Nikkor'. A removable lens can be converted into a macro
lens by the addition of an extension tube
A flat port, ie., a plane
glass or plastic optical window. The term arises because a flat port is
most useful with macro lenses.
: Methyl-ethyl-ketone. CH3
A volatile, flammable, chemical solvent. Used in the printing
(of Ni-Cd cells): Early Nickel-Cadmium cells had
a reputation for losing capacity if they were not discharged fully
during each cycle of use. This was dubbed 'the memory effect', i.e.,
the cell remembered that you didn't use it to the full and changed
accordingly. Improved design eliminated the problem, but the
myth persisted. NiCd batteries are now banned from sale to
ordinary consumers in Europe under the Restriction of Hazardous
Substances (RoHS) legislation (they are still permitted in emergency
lights for their tolerance of continuous trickle charging).
The modern replacement (NiMH) has little or no memory effect,
but cannot stand being overcharged and so requires a complicated
A lens that is concave on
one side and convex on the other. If the curvature of the
is greater than that of the concave part, the lens is a magnifying
glass. If the concave part has greater curvature than the
the lens is a de-magnifying glass. Inexpensive screw-in
are of the meniscus type, this being a good compromise between optical
quality (freedom from aberrations) and cost. Close-up lenses
constructed as achromatic doublets are considerably superior, but much
: Abbreviation for Motormarine Mk II (A
Sea & Sea camera).
The Motormarine II bayonet
mounting system for supplementary lenses.
: Dow-Corning trade name for
O-ring lubricants. Molykote 111 is the industry standard silicone
grease, a highly water-repellent non-toxic lubricant, often re-packaged
into small tubs by underwater housing manufacturers. Molykote FS1292 is
fluorosilicone grease, a special formulation designed to reduce the
problem of swelling due to grease absorption that occurs when silicone
grease is used on silicone rubber O-rings. Despite the name, Molykote
lubricants do not contain molybdenum disulphide.
: Modulation transfer function. A
factor between 0 and 1 indicating the extent to which contrast is
reduced when an optical system is projecting a pattern at a particular
resolution. Also known as Spatial Frequency Response (SFR)
Nitrile Butadiene Rubber. A rubber with excellent resilience and
chemical resistance. Preferred material for the manufacture of camera
O-rings, usually black. NBR has only limited UV resistance, and so
should be stored in the dark. The optimal lubricant for NBR is silicone
: New E-mount eXperience.
Sony interchangeable lens compact camera using the E-mount
lens bayonet specification.
battery. NiCd batteries are no longer marketed for consumer
applications in Europe due to RoHS regulations (Cadmium is no longer
permitted in disposable items). See also memory
Underwater film stills camera
system, manufactured by Nikon; developed from the 'Calypsophot' camera
designed by the Belgian engineer Jean DeWouters for the Calypso
expeditions of Jacques Cousteau.
: Petroleum gel (also
marketed as 'Vaseline'). Suitable lubricant for NBR (black nitrile)
O-rings, but somewhat inferior to silicone grease.
: There are three
generations of Nikonos connector: I (2 wires, used on Calypsophot,
Nikonos I and Nikonos II), III (3, wires, used on Nikonos III and IV),
and V (five wires, used on Nikonos V). Nowadays, only the 5-wire (TTL
flash) connector is used.
||Nikonos V (or 5)
Note that the socket has three fixed pins and two spring-loaded
retractable pins (this is for compatibility with the Nikonos III plug).
Some manufacturers produce a variant with fixed pins in place of the
retractable ones. This is not a true Nikonos connector, and a standard
Nikonos V plug will be damaged if it is screwed into such a socket.
: Nickel metal-hydride (cell or
battery). High-capacity rechargeable battery, free from toxic cadmium.
: see NBR
: New Old Stock
'National Television Systems
Committee' - The colour television system used in the USA, Canada, and
The unit of electrical resistance. A resistance of 1 Ohm will pass a
current of 1 Amp when a voltage of 1 Volt is applied across it.
: Own Equipment Manufacturer. Generally
a supplier who builds finished products from sub-assemblies supplied by
aye!"): Affirmative, Yes. A
light on a flash unit that illuminates briefly when a TTL-stop
(quench) signal has been received. See TTL
organic light-emitting diode.
: Abbreviation for 'O-ring'.
The un-prepossessing rubber ring
that keeps the water out of underwater housings is actually part of an
extremely clever sealing system that becomes more efficient as the
pressure difference between inside and outside increases. The
cleverness lies not so much in the ring itself, but in the shape of the
groove in which it sits, which is designed so that the pressure forces
the rubber into the gap through which the water is trying to pass.
Consequently, and perhaps surprisingly, an underwater housing is more
likely to leak at shallow depths than at great depths. One thing that
will disrupt the operation of an O-ring seal however, is dirt, which
creates gaps and channels through which the water may creep. Good
husbandry in underwater photography therefore, lies in the matter of
cleaning the O-ring and the O-ring groove meticulously before closing
A device that undergoes a
cyclical behaviour when supplied with energy. A pendulum is an
oscillator. A weight on a spring is an oscillator. The term however is
normally used to describe electronic circuits that produce an
alternating current or voltage when connected to a battery or a DC
power source. Oscillators are used in radios, TVs, watches, cameras,
electronic flash units, computers, etc., etc., etc.
'Phase Alternating Line' - The analogue colour television system used
Australia, and New Zealand.
The apparent difference in
position of an object when seen from two different viewpoints. The
problem with cameras having a viewfinder separate from the main lens
(as opposed to reflex cameras) is that the main lens and the viewfinder
see the subject from slightly different positions. This causes framing
errors. A partial solution is to tilt the axis of the viewfinder, but a
different amount of tilt is required depending on the distance to the
subject. External viewfinders sometimes have a parallax compensation
knob, which tilts the viewfinder according to a distance scale.
Built-in viewfinders are usually set for correct framing at some
specified distance, but may have parallax compensation marks in the
framing window for use at short lens to subject distances. Single-Lens
reflex cameras do not suffer from viewfinder parallax.
: Lead (plumbum).
: PalmCorder, i.e., a Camcorder that
can sit on the palm of the hand.
'Photo Co-ax.' Small circular
connector used in conjunction with manual flash equipment.
Printed circuit board.
: Pulse-code modulation. Digital audio
recording mode used by tape recorders.
: 'Picture element'. Now
preferred term is 'Pixel'.
(trade names: Terylene, Mylar). Clear injection mouldable and
recyclable polyester material, used to make drinks bottles, clothes,
insulation films for capacitors, etc.
: A set of words that represent letters, used
to facilitate the sending of messages via restricted-bandwidth
communication systems (i.e., radio). The International
is the approved set of such
words, being chosen so that no two words sound similar: Alpha, Bravo,
Charlie, Delta, Echo, Foxtrot, Golf, Hotel, Italy, Juliet, Kilo, Lima,
Mike, November, Oscar, Papa, Quebec, Romeo, Sierra, Tango, Uniform,
Victor, Whiskey, X-ray, Yankee, Zulu.
law, it is the Photogropher who will own copyright on any photos he/she
has taken, with the following exeptions
1. If the photographer is an employee of the company the photos are
taken for, or is an employee of a company instructed to take the
photos, the photographer will be acting on behalf of his/her employer,
and the company the photographer works for will own the copyright.
2. If there is an agreement that assigns copyright to another party.
for more info look at the Photography
Copyright fact sheet from UK Copyright Service.
: Programmable Interrupt Controller.
A microcontroller (single-chip computer) used for controlling
electronic equipment. An 'interrupt' is a request for action,
such as might be generated by a button-press or some other input.
of a colour display:
a dot or a small area of a picture which carries the three attributes:
hue, colour saturation, and brightness. Any picture can be made up of
an array of 'dots' carrying these attributes and viewed from such a
distance that the dots are no longer visible. In practice, pixels are
usually composite entities; the pixels of a TV screen, for example,
being made up of areas of red, green, and blue.
of an RGB image file
(after decompression): a set of three binary numbers representing
brightness values for red green and blue at a particular point in the
of a CMYK image file
(after decompression): a set of four binary numbers representing
amounts of cyan, magenta, yellow, and black ink to be applied at a
particular point in a printed image.
of a camera:
individual light-sensing element. The number of pixels attributed to a
digital camera is usually the total number of light sensing elements
(red + green + blue). The camera pixel count is usually taken to be a
figure of merit for image resolution (detail recording capability), but
this figure is not directly related to the displayable pixels recorded
in the output files.
For more information, see the article Pixels and
Professional Mobile Radio - e.g., the
hand-held VHF/UHF transceivers used for emergency and ship-to-shore
: Portable Network Graphics. File
format designed to get around the software patent on the LZW algorithm
used in GIF compression (Unisys patent on LZW expired in the US in
2003, but may still apply in some European countries, including UK).
: Price on application;
i.e., when someone asks for one we'll find out and let them know.
Tough plastic which can be used to make high-quality injection-moulded
engineering components. Trade names: Lexan, Merlon, and Tuffak.
Preferred material for sport-diving-depth underwater housings; also
famous as the material used to make bullet-proof shields. Polycarbonate
is attacked by chlorinated solvents, but can be cleaned using mild
detergent and water or alcohol. Polycarbonate is liable to crack if
drilled or machined incorrectly (specially ground drills are required).
See also Lexan
Optical quality window in an
underwater housing (by analogy with a ship's 'port hole').
Have you ever wondered why
people look extremely ugly in passport photos (particularly the ones
taken in photo booths)? The reason is that the picture has been taken
from short-range using a wide-angle lens, causing the subject's ears to
disappear behind the cheek-bones. To photograph a face in proper
proportion, the camera must be a reasonable distance away from the
subject (not possible in a photo-booth) and a lens of somewhat longer
focal length than a normal (standard) view lens is required in order
for the head and shoulders to fill the frame. For 35mm cameras, the
best focal length for portraits is in the range 80 - 135mm, and lenses
with focal lengths in this region are known as 'portrait lenses'.
: A short burst of light emitted
from a flash unit just prior to the main flash. A pre-flash can be used
for exposure evaluation: i.e., it can be used to make a trial exposure
from which the correct exposure for the photograph is calculated. One
or more pre-flashes can also be used to cause a portrait subject's
pupils to contract, this being a somewhat dubious attempt to reduce the
red-eye effect which plagues compact cameras (and which results from
placing the light source too close to the camera lens). Some cameras,
particularly when set to control proprietary slaves, may issue multiple
pre-flashes during the course of an exposure evaluation.
Photoshop document. Adobe proprietary
format for storing multi-layered images (ie., photo-montage in
progress). PSD is the preferred image file format for Photoshop users;
and given the widespread use of Adobe software in the photographic and
publishing industries, it is also a perfectly good file exchange
format. Note that the PSD specification changes with every whole number
version release of Photoshop, but is backwards compatible; so you must
have the latest version to be sure of being able to open any PSD file.
point from which all rays of light appear to emerge from a lens. Entrance
The point from which rays of light appear to emerge
from a lens when it is reversed (turned around). For compound
(multi-element) lenses, the precise location of the pupil is best found
by looking at the manufacturers detailed specification (if available),
but for practical purposes, it is where the iris appears to be when you
look into the lens. Note that for an asymmetrical lens, the pupil
appears to be in a different place depending on which side of the lens
you examine, for which reason we talk of `entrance' and `exit' pupils.
The entrance pupil is the one you can see from outside the camera.
'Poly Vinyl-chloride'. A normally hard plastic, with good resistance to
weathering and UV light, which can be made flexible by the addition of
plasticisers. The un-plasticised version is called "UPVC". PVC is
attacked by many industrial cleaning agents, especially chlorinated
solvents, ketones (e.g., MEK), and furans (e.g., THF). Cleaning with
agents other than mild detergent and water is not recommended.
Extinguish, put-out. The business of turning off the current in a
flash-tube for the purpose of automatic exposure control, it being no
easy matter to put the damned thing out once the discharge has begun.
In TTL flash photography, the camera must send a 'quench' signal to the
strobe (flash gun).
There are two ways of
quenching a photographic flash. The modern method is to switch off the
current in the tube using a series control element such as a GTO (gate
turn-off) thyristor (often the ubiquitous Mitsubishi CR3JM), or better
still an Insulated Gate Bipolar Transistor (IGBT). The old fashioned
method is to dump the residual charge in the capacitor by shorting it
with a special trigger tube (thyratron) with a very low on-resistance,
called a "quench tube". The quench tube is a small Xenon tube with a
large electrode area and a small inter-electrode distance. Quench tubes
waste the residual energy and require the system to be recharged from
scratch after a controlled flash. Switching off the current with a GTO
or IGBT conserves the residual energy and so shortens the recycle time
after a controlled flash. Consequently, quench tube systems are
obsolete (and very annoying to use).
The rectangular area of illumination formed on a TV screen by the
scanning process. An array of lines or dots of varying brightness
which, when viewed from a suitable distance, produces the illusion of
an image. Originally a German word for a printer's half-tone screen.
Probably first used in connection with 'electrical photography' by
Arthur Korn in his 1907 book "Elektrische Fernphotographie (electrical
distance photography)" (source: Richard F Lyon, Foveon Inc. private
The conversion of a
vector-based graphical representation into an array of pixels.
: A flash unit is ready to fire once
the main storage capacitor has been charged to a voltage at which the
tube can be expected to trigger reliably. This situation is usually
communicated to the user by lighting a neon lamp or an LED, and to the
camera by sending a signal along the sync. cable which causes the
camera to adopt a shutter speed at which flash (X) synchronisation is
possible. Thus a flash unit has a 'ready light', and a flash cable or
connector (with certain exceptions) carries a 'ready signal'. Note that
the ready light or signal is asserted when the flash unit is
triggerable, but it does not imply that the flash is ready to give full
output. If a flash unit has voltage regulation, you will hear
interruptions in the whining noise from the inverter when the capacitor
is fully charged. If the flash unit has no voltage regulation, the
inverter will run continuously, and in this case, as a rough rule of
thumb, you should wait about twice the time it takes for the ready
light to come on after a full light burst to ensure that the capacitor
is fully charged.
Rebikoff port, Rebikoff corrector:
which corrects for refraction at the air-water boundary, characterised
by a flat surface in contact with the water and a concave surface in
contact with the air. The Rebikoff corrector is afocal, and produces
less chromatic aberration than a dome port, but is more expensive to
Reciprocity, Principle of
exposure is defined as: light intensity multiplied by exposure time,
ie., I × t. It follows that a given amount of exposure can be
achieved by using a high intensity and a short time, or a low intensity
and a long time, or somewhere in between, provided that the required
product I × t is obtained. This is known as the
(photographic) principle of reciprocity (increase one, reduce the
other). The principle of reciprocity only holds true however if the
recording medium has a linear (proportional) response to light
intensity, and this is not always the case. Film is reasonably linear,
but needs additional exposure time at very low light levels. CCD
sensors become non-linear at very high brightness levels. When the
exposure setting (as determined using a light meter, for example) has
to be adjusted to compensate for very low or very high brightness
levels, the recording medium is said to suffer from reciprocity
: A lens which reproduces
right-angles and straight lines correctly.
: If a flash light source is placed
very close to a camera lens, a direct reflection from the blood vessels
in the back of the eye will cause a portrait subject to appear to have
illuminated red pupils. The proper solution to this problem is to move
the light source away from the lens (i.e., to use a flash unit separate
from the camera). The compact-camera solution however is either to
shine a light into the subject's eyes, or to fire one or more
pre-flashes; the object of the exercise being to cause the subject's
pupils to contract, thereby 'reducing' (but not eliminating) the
red-eye effect. All too often, a pre-flash will cause the subject to
blink; and we may then presumably console ourselves with the
observation that there is definitely be no red-eye problem when
photographing people who have their eyes closed.
The bending of rays of light
as they pass through the boundary between two optical media, e.g., from
air to water, or from air to glass. Only rays which are not
perpendicular to the surface are bent.
A measure of refracting
power, defined as the velocity of light in vacuum divided by the
apparent velocity of light in the medium. Water has a refractive index
of about 1.33, which means that light travelling through water has an
apparent velocity (phase velocity) of about 0.75 × the true
speed of light.
conductors (except superconductors) have some resistance, which means
that they convert electrical energy into heat. A resistor is simply a
device for which the property of resistance is accurately
characterised. Resistors are useful because they obey 'Ohms law', ie.,
the current which flows in a resistor is exactly proportional to the
voltage applied across it.
(resolving power): The detail
recording capability of an image acquisition system (camera, lens,
etc.). Resolution is usually expressed in lines per unit length, e.g.,
colour film can resolve about 110 lines/mm, the UW-Nikkor 15mm f/2.8
lens can resolve 73 lines/mm at the centre of the image at f/8. The
number of pixels in an image is not
a measure of
resolution, but it is related of the maximum amount of detail which can
Positive (i.e., slide)
film. Film in which the original negative image is reversed during the
development process to produce a positive transparent image.
Red, Green, Blue. The 'additive' primary colours. The colour-space
representation used by TV and Video monitors.
The square r
of the m
ean of the s
Of an alternating voltage or current: the equivalent steady (direct)
voltage or current which will have the same heating effect.
: Restriction on the use of Hazardous
Substances. European legislation.
"Societé de Constructeurs d' Appareils Radio recepteurs et
Téléviseurs". 21 pin interface connector for
direct audio + video connection between TV and VCR, DVD, etc. Also
known as a 'Péritel' or 'Euroconnector'.
Self-contained Underwater Breathing
Apparatus (as opposed to diving equipment supplied with air from the
surface). Term normally applied to diving equipment which supplies air
from a tank by means of a demand (suction operated) valve. Invented in
practical form in the 1940s by Jacques Cousteau and Emile Gagan.
Cousteau originally dubbed the device an "Aqualung"; but the British
Navy decided that this sounded like an unpleasant disease, and insisted
on renaming it "SCUBA".
Also known as a Nelson
connector. A 2-pin wet-connector, ie., a connector which can be
pulled-out and plugged-in underwater. Used for flash synchronisation on
the Sea & Sea Motormarine Mk1. Also used as a telephone
connector on full-face diving masks.
Sea & Sea connector:
waterproof connector used with the Sea & Sea TTL flash system.
'Sequential Couleur avec Memoire' -
The colour television system used in France, French overseas
territories (not Canada), and Russia.
Shore hardness, Sh:
A hardness scale from
0 - 100 used to express the deformability of elastic materials
(rubbers). Commonly available O-rings have a hardness of around 70sh.
The O-rings used in Nikonos cameras are 60sh. Using too soft an O-ring
may result in failure of the seal by extrusion (pushing-out). Using too
hard an O-ring will result in extreme stiffness, and potential failure
due to excessive wear.
: The time interval between
pressing the shutter-release button and the opening of the camera
shutter (often several tenths of a second). Some delay is inevitable in
SLR cameras (which must flip a mirror out of the way); and in cameras
which use the pre-flash method of flash exposure evaluation (and must
therefore allow time for the flash unit to recover from the pre-flash).
Auto-focus systems exacerbate the problem enormously, expecially when
working in low-light conditions, and there is considerable advantage in
having the option to lock the focus, or to use manual focus.
Système Internationale d'unités - the
system of weights amd
measures developed by the international scientific community and
adopted as the basis of all modern measurement systems. Also
known as the rationalised metre-kilogram-second (mks) system.
Slave flash, slave sensor
: A slave flash
is a flash unit which is triggered to fire by the light output from
another flash unit (the master flash). The light from the master flash
is detected by a sensor, usually a phototransistor, to generate an
electrical signal in the event of a sudden increase in light level. The
slave sensor can either be built-in to a flash unit with a 'slave
mode', or it can be a separate unit which triggers a flash via its
: Camera tray with captive tripod
screw in a slot, arranged so that the camera or housing can be pulled
back against an anti-rotation ridge.
Single-Lens Reflex Camera. A camera
which uses a mirror to redirect the image which will be projected onto
the film into the viewfinder. The mirror flips out of the way when the
button is pressed to take the picture.
: Single-lens semi-transparent-mirror
reflex camera (SLR in which the mirror stays in place during shooting).
: Tin (stannum).
Snell's Law, Snell's Window
: Energy transmitted by longitudinal
pressure waves in a medium such as air or water. The human hearing
range is generally considered to run from about 15Hz (cycles per
second) to 20KHz, although this view is somewhat misleading. Humans
generally feel rather than hear low frequencies, and the auditory
perception of sounds below about 50Hz is mainly due to the production
of distortion products (harmonics) by the ear itself. Only young
children can hear up to 20KHz, and by middle age, 12 to 15KHz is more
realistic (8KHz if you were in the habit of listening to loud music as
a teenager). The intelligence in human speech is contained in a band
from about 300Hz to 4KHz. Radio communication systems (PMR, Ship to
Shore) often limit the bandwidth to 300Hz to 3KHz, but this makes it
necessary to spell out some words using the Phonetic
, there being, for example, no way to distinguish
between the sounds for 'f' and 's'.
The velocity of sound in
dry air at 1Bar is given approximately by the formula:
v = 331.45 + 0.6T metres / second (where T is the
temperature in centigrade).
A figure to remember for general reckoning is: "about 340m/s".
For estimating the distance ot thunderstorms: "about 3 seconds per
If you need an accurate value see:
"The variation of the specific heat ratio and the speed of sound in air
with temperature, pressure, humidity, and CO2
concentration" Owen Cramer, Journal of the Acoustical Society of
America, May 1993, Volume 93, Issue 5, pp. 2510-2516.
The velocity of sound in sea water is approximately 1500m/s.
The velocity of sound in fresh water is approximately 1435m/s.
Sony / Philips Digital Interface
Format. Digital audio interface standard, for transferring audio
signals between CD players and DAT machines, etc., without passing into
the analog domain.
: Direct reflection, i.e., the glinting
reflections from white and shiny objects that cause bleaching of the
highlights in a photograph. The problem of specular highlights can be
reduced by using a diffuse light source, i.e., a light source of large
area, rather than a point source.
As opposed to bulb flash, a slightly old-fashioned name for electronic
flash. Term still used by Nikon and Canon, and a pleasant alternative
to the American term "strobe" which avoids using the words "gun" or
SS, Stainless Steel
: An alloy of Iron,
Chromium, Manganese, Silicon and Carbon (May also contain Nickel and
Molybdenum). The alloying elements, particularly Chromium, react with
oxygen and water to form a thin film of oxides and hydroxides, which
prevents further chemical attack. All stainless steels contain at least
10% Chromium. The most commonly available grades of stainless steel are
denoted 'A2' and 'A4'. A2 stainless (also known as type 304) shows
superficial rusting in the presence of sea water. A4 stainless (aka
type 316) is completely resistant to sea water corrosion and is the
preferred grade for marine applications.
Photographers talk loosely of
relative illumination level in 'stops'. A change of exposure level of
one stop is a doubling or halving of the shutter speed, a doubling or
halving of the ISO/ASA film speed, or a doubling or halving of the area
of the lens aperture, i.e., in the latter case, a change of one f-stop
which the term is derived. A change in illumination of 1 stop is the
same as a change in illumination of 1 'Exposure Value (EV)', this being
the correct term.
The terms `Strobe Light' and
`Flash Gun' are interchangeable. The former usage is North American,
the latter originates from the UK. It is best to avoid the common
British term when travelling outside the UK since, if some official,
who may not speak English very well, asks what you've got in your bag,
you don't really want to use any phrase with the word 'gun' in it.
In British English however, 'strobe' carries the connotation
of repeated flashing (it is a shortened form of 'stroboscope'), so use
of the term can cause confusion. International English
alternatives are 'photo flash', 'flash unit' and 'speedlight
', but not 'Blitz'
(lightning), which has the additional British English meaning 'wartime
for an underwater flash unit.
Super VHS, i.e., High-band
'Separated Video', ie., two
signals in two cables, one being the video, blanking and
synchronisation, the other being the chrominance (colour) signal.
Sync., synchronisation (flash)
: see X-sync
. The cable between the
camera and the flash unit is called a 'sync. cord'.
A television picture is
composed of lines, of variable brightness and colour. A television
monitor writes horizontal lines on the screen, starting at the top
left-hand corner and travelling downwards. Synchronisation signals are
required to tell the monitor when to start a new line (horizontal
sync.), and when to flip back to the top for the start of a new field
(vertical sync.). The sync. signals are in the form of electrical
pulses which are added to the video (picture) signal.
System compact camera
: A camera which has
interchangeable lenses but which is not an SLR. Eg., Nikonos,
A light, usually built into a flash unit, used to give an accurate
indication of the direction in which the flash unit is pointing. The
targeting light should be designed to be weak in comparison to the
intensity of the flash, so that its patch of illumination does not show
up in the picture. When using a torch as an auxiliary targeting or
focusing light, beware that some modern torches are far too bright for
the job and may ruin your pictures.
Abbreviation: 'To be announced'.
Time-base corrector. Helical-scan
analogue video recordings (Video8, Hi-8, VHS, etc.) are affected by
minor speed fluctuations (wow and flutter) in the tape-transport
mechanism. This causes timing jitter in the horizontal synchronising signals
, and leads
to wobbly verticals in the picture. A time-base corrector digitises the
video signal and stores the lines temporarily so that they can be
released at exactly timed intervals, causing a dramatic improvement in
: A problem with digital
camera sensors is that they separate colours by placing an array of
colour filters in front of an array of light sensitive cells. This
means that a light ray striking the sensor at an oblique angle can leak
across into an adjacent cell, causing a degradation of the colour
analysis (i.e., 'colour crosstalk'). The problem is minimised by
placing the exit pupil of the lens at infinity (in front of the lens),
in which case the lens is said to be image-side telecentric. The result
is that light rays emerge substantially parallel to the optical axis.
The lack of
telecentric design is one reason why old wide-angle lenses designed for
film cameras tend to give poor results with digital cameras. Standard,
portrait and telephoto lenses generally do not to send severely
off-perpendicular rays to the sensor, and so old lenses of longer focal
length are often still capable of good performance.
: Tetra-hydro-furan. A volatile,
flammable, chemical solvent; used as the basis for PVC and ABS
Tagged Image File Format. A lossless
full-colour image file format with optional LZW compression. TIFF files
can be opened, edited and re-saved without degrading the image. The
TIFF specification allows for up to 48 bits-per-pixel, CMYK or RGB
colour, and covers just about every requirement for electronic
representation of single-layer still images. TIFF is the format of
choice for exchanging and archiving high-quality photographs.
Technical Lighting Control - Aquatica
trade name for a range of lighting support brackets and adapters.
An electronic switching device
which (among manifold other uses) is used to switch off the current in
a flash tube for the purpose of TTL exposure control.
An electrical device for
changing from one AC voltage to another, usually consisting of two or
more coils of wire wound on a common core of magnetic material. Used in
electricity substations for transforming from the distribution-grid
voltage to the local mains voltage. Used in power supplies and battery
chargers for transforming from the mains voltage to the low voltages
required by electronic equipment. Used in conjunction with an
oscillator, for generating high voltages from low voltages inside TV
receivers and flash units. A device which comes in all sizes and has a
huge number of uses (the smallest transformers are about the size of a
grain of rice, the largest are delivered on huge low-loaders with
The metal plate or bar which screws
to the bottom of a camera or housing so that a supporting arm for a
flash unit or lamp may be attached to it. Also, the metal or plastic
plate on which a camera sits inside an underwater housing. Sometimes
also called a 'saddle', a 'stay', a 'rail', or a 'bracket'.
'Through The Lens' flash metering. Another term, which never caught on,
is 'Off The Film' (OTF) flash metering. It is basically an improved
form of automatic flash control which takes account of the film speed,
lens aperture setting, and any filters which might have been fitted to
Although the full
burst of energy from a flash unit may seem instantaneous, it usually
last for about 1.5 thousandths of a second (~1.5ms). The light output
can therefore be controlled by reducing the time duration of the flash,
ie., by switching off the current in the tube before the storage
capacitor is fully discharged. A TTL flash unit is one which can accept
a signal from the camera telling it to 'switch off now'. The important
thing to understand about this system is that the flash unit plays no
part in calculating the exposure; it simply accepts the instruction to
switch off at a point determined by the electronics in the camera, and
will usually issue some kind of error signal if no such instruction is
received. In 'OTF' metering, the camera determines the exposure by
measuring light reflected from the film and summing (integrating) it
over time. It compares the amount of light received against a level
determined by the ISO (ASA) setting, and issues a stop (quench) signal
at the appropriate point. If you reduce the amount of light falling on
the film by stopping the lens down or fitting a filter, the camera will
take longer to issue the stop signal. The camera can therefore
compensate for all relevant variables until a point is reached when
there is insufficient light to complete the exposure; in which case no
stop signal is sent, and the flash unit issues a warning (Sea &
sea strobes have a green 'TTL OK' light which comes on if a quench
signal is received. Nikon and Ikelite strobes flash the red 'ready'
light if no quench signal is received.). It follows, that a TTL flash
system requires a TTL capable flash unit and a TTL capable camera; and
less obviously, that the two units must be compatible (ie, they must
conform to the same electronic interface standard).
With the advent of
digital photography, the definition of 'TTL flash' has expanded to
include the 'pre-flash' method of exposure evaluation. It is difficult
to devise a good 'off the film' metering system when using a digital
sensor, because the sensor has a shiny rather than a matte surface. The
solution is to issue a pre-flash, determine the exposure level obtained
by averaging information read from the CCD, and use this to calculate
the required duration for the main flash. The same system of start and
stop (trigger and quench) signals can be used to control the flash
unit, provided that the flash unit can recover and be ready to fire
again in the short (less than 100ms) interval between the pre-flash and
the main flash. A disadvantage of the pre-flash system is that it
increases shutter lag (i.e., the time between pressing the button and
taking the photograph), and it may cause portrait subjects to blink.
Improvements in flash technology (the use of an IGBT as the switch-off
device), can reduce the pre-flash - main flash interval to about 30ms.
Light metering for the
determination of photographic exposure can be carried out in two ways:
either by measuring the ambient light; or by measuring the light
reflected by the scene. The ambient light method is best for
reproducing the overall ambience, whereas the reflected light method is
best for fitting the range of brightnesses encountered to the contrast
range of the recording system (film or sensor). Nowadays, the reflected
light method is almost universal because it can be done 'through the
lens' i.e, using a light sensor inside the camera, or using the image
sensor itself. This has the advantage that the reading can be used to
adjust the exposure settings automatically, and it automatically
compensates for the light loss which occurs in lenses and filters. No
exposure metering system is perfect however, especially if the scene
has very high contrast; for which reason good cameras have exposure
compensation controls and allow the photographer to examine the image
Ultralight Control Systems. Manufacturer of lighting brackets.
Sony analog video cassette
recording system using helical scanning and 3/4" magnetic tape. First
appeared mid 1970s. Once favoured by industrial training departments,
drama schools, etc., but now obsolete.
Unified Coarse. An American system of screw-thread sizes based on inch
dimensions but with the same 60° pitch angle as the European
metric thread system. The most common UNC thread used in photography is
1/4UNC (the standard tripod socket thread) more completely specified as
1/4UNC20, i.e., a 1/4" (6.35mm) diameter rod threaded at 20 turns per
: Unified Extra-Fine.
: Unified Fine. American fine thread
system with 60° pitch angle. Smaller UNF sizes are numbered,
whereas larger sizes are known according to the nominal diameter. E.g.,
#10UNF32 = 0.19" diameter rod threaded at 32 turns per inch. 1/4UNF28 =
1/4" rod threaded at 28tpi.
a hard engineering plastic. See PVC
Universal Time Co-ordinate, i.e., the
astronomical time at zero longitude. Formerly known as GMT (Greenwich
Beyond violet. That part
of the electromagnetic spectrumwhich lies beyond the violet end of the
visible spectrum, ie, of shorter wavelength than violet, and invisible
to humans. The problem with UV is that it will pass through camera
lenses, film and CCDs are sensitive to it, but lenses are not designed
to focus it. The result is a haze or fuzziness in pictures taken
outdoors. The solution is to fit a UV filter (sometimes called a
UV-390) ie., a filter designed to cut off at a wavelength of 390nm
(nanometres), the end of the visible spectrum. Some photographers fit a
UV filter also to protect the main camera lens from scratches. A UV
filter is not needed underwater.
Correction (filter). An optical filter designed to introduce a colour
cast having the complementary colour
of the cast introduced by the water. The colour cast introduced by
so-called 'blue' water is somewhere between blue
hence the complementary colour is somwhere between yellow
The complementary colour for green
water is magenta.
Videocassette recorder. See also VTR.
: Video Home System; i.e., a video
recording format intended primarily for domestic use. Analog recording
system using helical scanning and 1/2" magnetic tape.
: VHS Compact. Small-size VHS
cassette intended for camcorders. Can be played back in full-size VHS
machines by means of an adapter.
Latin: 'I look'. An electrical
signal which represents a picture; as opposed to television: a
modulated radio signal which represents a picture. A video standard
describes the details of the basic electrical signal which passes along
local cables. A television standard is an extended video standard
containing details of a method by which video signals and accompanying
sound can be transmitted by radio and other long-distance communication
Analog video recording system
using helical scanning and 8mm magnetic tape.
Volt, Voltage, V:
The unit of electrical
pressure (strictly: pressure difference), named after Henry Volta who
invented the 'Voltaic pile' (nowadays known as the battery). Since a
voltage is a pressure difference
, it must always
be measured relative to something, and that something is usually a
common connection called 'ground', 'earth', 'common', 'chassis', or
Videotape recorder. A videocassette
recorder (VCR) can also be called a VTR, but a video recorder which
uses open-reel tapes cannot be called a VCR.
Double (because it's a "double U"). Used as a prefix of suffix in
Japanese product names, eg: "W-Stay" = dual flash lighting tray;
"Lens-holder W" = dual lens holder.
: Tungsten (wolfram)
The unit of power, ie., the
amount of energy dissipated (used) or delivered in unit time. 1 Watt =
1 Joule per Second. Electrical power, in Watts, is also the product
Current × Voltage (P = IV), eg., a 12V light-bulb which
consumes 60W must draw a current of 5A.
see Electromagnetic radiation
: Waste Electrical and Electronic
Equipment directive. European legislation.
A connector that can be un-plugged and re-connected underwater.
Sealock (Nelson), E/O (Electro-Oceanic), and ICS-2 (Ikelite 2-pin) are
examples of wet connectors. Typically used for low-voltage and signal
connections, i.e., telephone, flash triggering, and video.
(Whit., BSW). Obsolete British
Standard screw thread system based on a 55° pitch angle. The
standard camera tripod socket thread was originally ¼" BSW
the 20 refers to the number of threads per inch (tpi). It so happens
that screws and sockets conforming to the the American
¼UNC20 specification (60°
angle) will fit with ¼BSW parts, and so the ¼UNC
thread is now the
de-facto standard for tripod attachments.
A wide-angle lens is one
which has an angle of view significantly wider than a normal human
perspective. Humans actually have a horizontal field of view of about
180°, but can only see clearly in the central field, which
corresponds to about 40°. To complicate matters, lens
manufacturers like to quote the angle of view corresponding to the
diagonal of the film format (because this gives a larger number), and
the effective angle changes when a supplementary lens is added. As a
rough guide, a `normal perspective' or `standard' lens for use in air
has a focal length approximately equal to the width of the film format
multiplied by √2 (i.e., W × 1.414). E.g., for the
35mm format (36 × 24mm), the standard lens is 36 ×
1.4 = 50mm. For square medium format (57 × 57mm), the
standard lens is 57 × 1.4 = 80mm. For a video camera, a
typical format might be 6.4 × 4.8mm, giving a normal view
when the zoom lens is set to about 9mm. If you use an `air' lens
underwater, note that the optical port counts as a supplementary lens,
and that a flat port reduces the effective angle of coverage.
Consequently, the standard focal length for an underwater 35mm camera
(assuming a flat port) is taken to be 35mm (although the logic behind
this convention is not strictly correct, since it corresponds to the
`normal view' of someone who can see underwater without a diving mask).
Any lens with a focal length longer than standard is called a
`Portrait' or `Telephoto' lens. A lens with a focal length shorter than
standard is a wide-angle lens.
Sanyo trade name for combined video and stills cameras.
: (pronounced 'zee-non' or
'zen-on') A member of the
family of gases known as noble gases (Helium, Neon, Argon, Krypton,
Xenon). A photographic flash tube is filled with Xenon at
than atmospheric pressure (usually between about 0.1 and 0.5 bar). The
Xenon gas electric-discharge photographic flash was invented in 1931 by
Prof. Harold Edgerton of MIT. Edgerton travelled with Jacques
on the Calypso expeditions, in which context he was known as 'Papa
That part of the electromagnetic
spectrum above ultraviolet in frequency (ie., of shorter wavelength
than UV, but not as short as Gamma-rays).
Electronic-flash synchronisation, as opposed to bulb-flash
synchronisation. In electronic flash photography, the light discharge
is very rapid, so (in the context of a mechanical film camera system)
the camera shutter must be fully open before the flash unit is
triggered. In cameras with a focal-plane shutter (a shutter which lies
right in front of the film) the shutter does not open fully when high
shutter speeds are used. Instead, the second-curtain of the shutter is
released before the first-curtain has finished travelling, and the film
is exposed by a slit passing across it. Consequently, X-synchronisation
is not possible above a certain speed. The highest speed at at which
electronic flash synchronisation is possible for a given camera is
called the 'X-sync. speed'. Synchronisation is, of course, possible at
slower speeds, but the effect of ambient light becomes more and more
important as the speed is reduced, and at some point, the exposure due
to ambient light may exceed that obtained from the flash. In modern
camera systems, a 'ready' signal from the flash is used to set the
camera to the X-sync. mode if the user tries to select a higher speed.
In digital systems, the 'shutter' is virtual, i.e, the exposure is
managed by means of a start and a stop signal sent to the sensor, and
management of flash synchronisation is done by the system
microcontroller and does not usually need to be considered by the user.
and Chrominance, i.e., the 'brightness information' and 'colour
information' parts of a composite video signal. The symbol Y was chosen
long before the computer age, when TV and printing technologies were
unrelated (the potential confusion being that Y also stands for
|YS: Yellow Submarine (we
all live in one). Beatles inspired name for Sea & Sea's first
ever product (a flash unit). Sea & Sea strobes are still called
"YS", even though most of them aren't yellow any more.
|YS-mount: The attachment
between Sea & Sea, Epoque, Sony and Olympus flash-units and
lights and the lighting support bracket (arm). Fixing-bolt thread is
M8. Distance between clamping cheeks is 12mm.
Y=Luminance, U=Normalised B-Y,
V=Normalised R-Y. The colour space representation used in PAL video and
North Americans pronounce the letter "Zee", and certain abbreviations
such as E-Z P-Z (easy-peasy) only make sense when you use the American
: Mounting attachment on Inon flash
units. M6 socket with facility for an anti-rotation pin.
© D W Knight, 2001 - 2011, 2018