Underwater SLR housings usually have interchangeable lens ports.
Some compact camera housings also have interchangeable ports.
Potential users of such housings must select an appropriate
lens and port combination in order to arrive at an optical system that
fulfils requirements. In general, manufacturers offer a
variety of flat ports, dome ports, extension rings, ports with
additional controls, and mechanical linkages (gears etc.); and this
article gives a brief summary of their properties and applications.
Sea & Sea SLR ports
Choice of lens
Water absorbs light, adds a colour cast and contains suspended
particles. A general rule for underwater photography is
therefore that pictures should be taken from as short a range as
possible (within reason) unless special considerations dictate
otherwise. This means that large objects and general scenery
should be photographed using a wide-angle lens if possible, and small
objects should be photographed using a close-focusing macro lens.
Macro lenses of 50-60 mm focal length (35 mm equivalent) are
best for photographing sedentary subjects because they place the least
amount of water between the lens and the subject. For
photographing fish and other animals which are likely to be disturbed
by the proximity of the camera however, macro lenses of around 100 mm
focal length are to be preferred.
Note that, although it is best to be
close to the subject, this does not mean that there is no practical
limit on the desirable angle of coverage for a wide-angle lens.
The best choice for general underwater photography is to use
a lens system with coverage in the range of about 75 to 100°.
Beyond that lies a special realm of photography in which
pronounced optical distortion occurs and effective flash illumination
is difficult to achieve. This means that if you buy a
super-wide lens system you should do so for a purpose, such as
photographing whales or shipwrecks by natural light, or to use fisheye
distortion for artistic effect.
Normal practice is to use flat ports
for macro and other lenses of relatively long focal length, and to use
a hemispherical dome port for wide-angle lenses. The optical
reasons for those choices are discussed below. Note that
ports, in general, do not have anti-reflection coatings.
Consequently, under some lighting conditions, the camera may
pick up a reflection of any writing or markings on the front of the
lens. The solution is to go over any white or light-coloured
markings with a black alcohol-based (permanent) felt-tip pen.
Inking out the lens markings also improves performance when
using ordinary filters and diopters above water. If you want
to restore the lens to its original condition, you can remove the ink
with cotton buds and alcohol (either industrial methylated spirit or
Flat (Macro) Ports
A flat port used underwater does not simply act as a window.
Instead, the air-water boundary causes it to act as a lens.
The effect of this compulsory extra lens element is to reduce
the angle of coverage and increase the magnification provided by the
main camera lens. The extra magnification is useful with
macro lenses, which is why flat ports are also called macro ports.
It also means however, that if (for example) you want the
same angle of coverage as a 50mm lens, you must fit a 35mm lens.
A table showing the reduction in coverage for a lens of a
given focal length is given in the angle
Flat ports are not ideal for use with
wide-angle lenses for several reasons: Firstly the port is likely to
cause vignetting (cutting off the picture at the corners); secondly the
port may introduce noticeable pincushion distortion (although this may
be offset by barrel distortion of the lens); and thirdly the
edge-sharpness of the picture will begin to deteriorate as the angle of
coverage increases. The loss of sharpness is caused by
chromatic aberration (colour fringing in off-centre detail).
Assuming a 36 × 24 mm picture format, or 35 mm
equivalent focal lengths, a general rule-of thumb is as follows: With
50 mm or longer lenses, chromatic aberration is negligible. A
35 mm lens will give acceptable results with a flat port, but is
noticeably better with a dome port. A 28 mm lens with a flat
port will give poor results. Chromatic aberration and
pincushion distortion are however completely reversible using radial
software; which means that, with a little extra
post-processing, sub-optimal lens-port combinations can be
used without penalty except for the loss of coverage (provided that
there is no vignetting).
Chromatic aberration of a 50mm lens and flat port
Aquatica housing with macro port
Subal housing with macro port
Shown below are 3 images scanned from a 35 mm (36 x 24 mm)
The photograph was taken underwater using a Sigma 50 mm macro
lens and a flat port, and was recorded on Kodachrome 25 film (the
picture was taken a few years ago, but the laws of physics remain
unchanged). The camera to subject distance was about 0.6 m.
The first image gives an overview of the photograph, the
second image is a detail from the exact centre of the picture, and the
third image is a detail from the top left-hand corner.
1) Complete photograph (red squares showing location of the pictures
2) Centre detail scanned at 126 pixels / mm (3200ppi).
3) Top left corner detail scanned at 126 pixels / mm (3200ppi).
There is no colour fringing in the centre of the picture, but the
corner image shows that detail in the blue colour channel is displaced
outwards, and the red colour channel is displaced inwards (relative to
the green channel), i.e., the magnification due to the lens-port
combination is greater at the blue (short wavelength) end of the
visible spectrum than it is at the red (long wavelength) end.
This is classic chromatic aberration due to an underwater
port. For pictures recorded using a 50 mm lens (35 mm
the effect is noticeable to the trained eye in large prints, but is
negligible for most purposes. As mentioned earlier, such
aberration can be corrected in software (see the image radial
Serious macro photography is done with a proper close-focusing macro
lens rather than with the 'macro' setting of some general-purpose zoom
lens. Macro lenses tend to change length somewhat radically
as the focus setting is adjusted, and so it is important that the
length of the port stem is chosen so that the lens cannot crash into
the optical window. Manufacturers produce ports of varying
stem length, or extension rings, to allow for this requirement.
One potential problem with macro
lenses and long ports is that the lens will be a long way back in the
port when set to focus at infinity. Hence it is not always
possible to prevent vignetting when the lens is to be used for
normal-perspective photography. This is a particular problem
of 1:1 macro lenses of focal length in the 50-60 mm range.
Nowadays, many photographers are
content to use auto-focus for macro photography and are perfectly happy
with the results. AF is not necessarily optimal in this
application however, firstly because focusing may be unreliable due to
lack of light (an auxiliary focusing light is recommended) and secondly
because the depth of field in macro photographs is often very limited.
When photographing a three-dimensional object, it is best to
focus on a point one third into into its depth, whereas an AF system
will tend to focus on the nearest detail. Hence, you may
prefer to use manual focusing, in which case it will be necessary to
fit a gear or clamp around the focusing ring of the lens and connect it
to a focus control. Since macro lenses are often physically
long, the focus ring may be out of reach of the control actuators
provided in the camera housing. For this eventuality,
manufacturers produce ports with an auxiliary focus control.
housing with Macro port
Port With Control
Dome ports were introduced in the 1960s, by the photographer Flip Schulke
others, and provided a solution to the problem of vignetting with
fish-eye lenses. It soon became obvious however, that the
dome port has another desirable property, which is that it corrects for
the reduction in angle of coverage caused by the air-water boundary.
A dome port is set-up for wide-angle photography by placing
the entrance pupil of the lens at the centre of curvature of the dome
(ie., the point at the middle if the dome were a complete sphere).
In this case, light rays heading for the entrance pupil
always strike the port surface at right-angles, no refraction occurs,
and the angle of coverage of the main lens is conserved. This
convenient situation comes at a price however, which is that the port
acts as a strong de-magnifying lens.
Based on an original illustration by Gale Livers of Ikelite
The effect of the dome port is to make the subject appear much smaller
and much closer than it really is, and the camera has to be able to
focus on this 'virtual' image. This is not a problem with
true wide-angle lenses, which have a very short minimum focusing
distance, but with moderate wide-angle (28 & 35 mm) and
lenses, it may be necessary to add a close-up lens to the front of the
main lens, especially when using a dome of small-radius (see below).
Needless to say, when using any underwater port, the distance markings
on the lens are not to be believed.
Dome Radius or Diameter
Some manufacturers use a nomenclature which attributes a number of
"inches" to a dome port. This measurement is not the diameter
of the port assembly, but is the inside diameter of the complete sphere
from which the dome is notionally cut. Hence an 8" dome port
has a 4" radius of curvature, measured from the centre of the notional
sphere to the inside surface of the dome. Similarly, a 6"
dome port has a 3" radius of curvature. Since the angle of
coverage of the camera lens is preserved when the entrance pupil (the
place where the iris appears to be when looking into the lens) is
placed at the centre of curvature, the radius of curvature is the
preferred distance from the entrance pupil to the inside surface of the
dome. When purchasing a dome port, it is necessary to obtain
something close to the required distance by selecting a port with the
nearest appropriate stem-length, or by choosing an extension ring of
the appropriate length.
The proximity of the virtual image
created by the dome is a function of the radius of curvature.
Small radius domes place the virtual image closer to the
camera than large radius domes. Hence a lens used behind a
small radius dome is much more likely to require a supplementary
close-up lens than a lens used with a large radius dome. The
use of close-up lenses is best avoided if possible, but large radius
domes have the disadvantage that they are delicate and vulnerable and
contain a lot of air (i.e., additional ballast weight might be needed).
Hence, instead of apportioning funds to the purchase of a
large dome, it might be better to use a small dome and use the money
saved to buy a lens with a small minimum focusing distance.
As a rough rule of thumb (based on the
approximation that the refractive index of water is 4/3, and the dome
is infinitely thin), a dome port of radius r will place the virtual
image at a distance 4r from the centre of curvature. The
centre of curvature is where the entrance pupil of the lens should be
placed, but the lens distance markings and minimum focusing
specification usually relate to the focal plane (i.e., the location of
the film or sensor, sometimes marked with the symbol
the lens needs to be able to focus at 4r + b, where b (the back focal
distance) is the distance from the entrance pupil to the focal plane.
The entrance pupil position may be obtainable from the lens
documentation, but can otherwise be estimated by noting where the iris
appears to be when looking into the lens. Thus, if we have
(say) a nominal 6" dome port with r=75 mm, then the virtual image in
water will be at about 300 mm in front of the lens entrance pupil.
If b is (say) 50 mm, then the lens will need to be able to
focus at <0.35 m if the system is to be able to bring light from
infinity to a focus. In practice, of course, we also want to
be able to focus on objects which are closer than infinity, and so
0.35 m minimum focusing distance is an upper limit (and the finite
thickness of the dome material actually brings the virtual image about
10 to 15 mm closer).
If a diopter lens is needed, note that
the number of diopters is defined as the reciprocal of the focal length
(of the diopter lens) in metres. Thus, if a dome port puts
the virtual image at 4r from the centre of curvature, then a magnifying
lens of 1/4r diopters placed at the centre of curvature (or
thereabouts) will make the virtual image reappear at infinity.
Thus a lens with poor minimum focusing distance used behind
our example 6" (r=0.075 m) dome port will require a 1/(4 ×
0.075) = 3.3 diopter lens screwed into its filter ring (a
slight under-correction is acceptable; so 3 diopters will probably
suffice, but more may be needed if the dome is thick or the diopter
lens is a long way in front of the centre of curvature).
Virtual image position and infinity restoration
(Approximation based on infinitely thin dome and refractive index of
water = 4/3. Actual virtual image for typical port will be 10
to 15 mm closer to the camera.)
to virtual image
For an accurate formula for the virtual image position, and a
calculation spreadsheet, see the Dome
Assuming that the main camera lens is perfect (and therefore also
mythical), a small radius dome port introduces more chromatic
aberration (colour fringing in off-centre detail) than a large radius
dome. An improvement in optical quality is not guaranteed by
changing to a larger dome however, because the aberration introduced by
the small dome may just as easily cancel some of the aberration of the
main lens as increase it. Assuming that a close-up lens is
not required, the lens data needed to determine which radius of port
will give the sharpest pictures with a given lens is difficult to
obtain, so it is probably best to choose the port radius on
practicalities (size, buoyancy, vulnerability) rather than optical
subtleties such as this.
Aquatica housing with 8" dome port
Ikelite housing with 6" dome port #5503
Ikelite 6" dome port #5503
Ikelite extended 6" dome port #5503.80
Ikelite housing with 8" dome port #5510.82
If a camera housing with a dome port is used above water, the apparent
position of the object will be very little affected by the presence of
the dome. This presents a problem when taking half-in
half-out photographs, because the camera must be able to focus both on
the normal object above water and on a nearby virtual image of the
object below water. The large dome has a clear advantage in
this situation, because it ensures that the virtual image is as far
away as possible. A large depth of field is still required
however, so half-in half-out photographs are best attempted using a
wide angle lens operated at a small aperture.
Use of Zoom Lenses
Note that, because a flat
port does not preserve the angle of coverage of the camera lens, a
half-in half-out photograph taken using a macro port will show the
underwater part of the image magnified relative to the above-water
part. Shown right is a half-in half-out picture taken using a
50 mm lens behind a flat port. The effect obtained is
generally considered to be undesirable, which is why a dome port is
Wide-angle zoom lenses of limited zoom range can be used behind a dome
port in much the same way as a fixed focal-length wide angle lens.
In such cases the zoom ring is fitted with a gear and
connected to a control provided on the camera housing, and the increase
in versatility in comparison to the use of a fixed focal-length lens is
For longer focal length "standard"
zoom lenses (e.g., 28-70 mm), the optical situation is usually not so
good. Some people like to use these lenses because they have
a built-in 'macro' facility and seem to offer everything in one
package. The problem however is that most of these lenses
change length sustantially as the zoom control is rotated, and are
physically shortest at the wide-angle setting. The port
length has to be chosen to allow for the lens at its maximum extension,
but then the front of the lens will be a long way from the port at the
wide-angle setting and vignetting will result. The solution
to the vignetting problem is to use a dome port, but then the optimum
magnification will not be obtained in macro mode. Wide-range
zooms also tend to have poor minimum focusing distances, which means
that macro photographs have to be taken from some distance (resulting
in foggy pictures), and the lens may not be able to focus on the
virtual image produced by the dome underwater. It then
becomes necessary to add a close-up lens to the front of the zoom lens,
which reduces the lens resolution slightly and prevents the system from
focusing above water.
Some zoom lens designs are physically
longest when set for widest-angle. These lenses can often be
used to good effect with a flat port (bearing in mind chromatic
aberration at the wide-angle end), but care is needed in selection of
the correct port length.
Most housing manufacturers provide
only a rotary coupling to the lens zoom barrel. Hence
push-pull type zoom lenses cannot be used (except perhaps by
pre-setting the focal length before closing the housing).
© Dave Knight & Steve Knight. 2006 - 2011,
2012. Updated Feb. 2018.