Optics & Photography

Starting out in Underwater Photography
Underwater housings, pressure, and swimming underwater.

You don't need to be an expert photographer or a SCUBA diver to take underwater photographs or use a video camera underwater; but if you're new to the game, this section might help you to understand what all of the equipment does and give you an idea of how to use it and look after it.

Any piece of electronic or delicate mechanical equipment destined to be used underwater requires a housing, i.e., a sealed box or casing designed to prevent the water from getting in. The housing can either be part of the equipment, such as the casing of a waterproof camera; or it can be a separate item designed to enclose a piece of equipment that is not otherwise waterproof. There are two basic types of housing: flexible housings, and hard (or pressure-resistant) housings. In the examples shown below; 'A' is a compact camera inside a pressure-proof housing, 'B' is a waterproof camera with its own integral pressure-proof housing, and 'C' is a camera inside a flexible housing.

ikelite Ultra-compact housing
Canon D10
ewa-marine housing

So, what's all this about pressure?
If you take a rigid box underwater and then make a small hole in it, the water will not dribble in, but squirt in, with a force that depends on how deep you are. This phenomenon is due to the weight of the water above you. On the surface of the planet, we are always under pressure due to the weight of the atmosphere above us, but we don't usually notice it because the air-spaces inside our bodies are at much the same pressure as the air outside. The only times when you might notice the effects of changing air pressure are when driving from the top to the bottom of a large hill, or in an aeroplane that is coming in to land; in which case, your ears start to feel strange until you yawn or move your jaw about. This is because, unless you have a very bad cold, the body has only two air-spaces that are not normally open to the outside, and these are behind the ear-drums in a place known as the 'middle ear'. The middle-ear has a tube leading to the throat, called the 'Eustacian tube', which is opened and closed by a small muscle called a 'dilator muscle'. The dilator muscle operates when you yawn, to keep the pressure on both sides of the ear-drum approximately the same so that you can hear properly. You'll need to make friends with your Eustacian tube dilator muscles when you learn to dive or snorkel.
     When you dive underwater, the problem of increasing pressure is much more severe than when you lose height in the air, because water is much denser than air. The air pressure varies slightly due to the weather, but on an average day, the pressure of the atmosphere, due to the weight of the column of air between the surface of the earth and interplanetary space, is only the same as the pressure due to about 9.9 metres of water. It's not so much the absolute pressure we're interested in however, but the pressure difference between the inside and outside of any box or vessel (or ear-drum).
     When you close a rigid box on the surface, the pressure inside is fixed at about atmospheric pressure. For convenience, we can refer to this pressure as 'one atmosphere'. When you swim down to a depth of about 10 metres, the pressure inside the box is still only at one atmosphere (or even less if the water has caused the air in the box to cool down) but the pressure outside is 2 atmospheres, i.e., the pressure due to the atmosphere plus the pressure due to the water. The pressure difference however, between the inside and the outside of the box, is one atmosphere. In general, the pressure difference increases by approximately one atmosphere for every 10 metres (10m) of depth, i.e., at say 30 m, the pressure difference between the inside and the outside of the box is about 3 atmospheres, but the absolute pressure (water + air above) is 4 atmospheres.
     The above examples were chosen for their nice round numbers. There is nothing magical about the pressure of one atmosphere, and it takes only a tiny fraction of that to cause a housing to leak. In fact, the cooling effect of plunging a housing into water (which makes the air inside want to contract) is enough to cause a leak without going to any depth at all. Fortunately, it is down to the engineers who design underwater housings to solve this problem for you, but given the enthusiasm with which the water wants to get in, the engineers can't help you if you don't take the business of closing the housing seriously.

Flexible housings
One solution to the pressure problem is to make the housing flexible. The housing can then simply contract (reduce in volume) to keep the pressure on the inside and the outside the same. This is a very reliable system, because the lack of any pressure difference means that there is no real tendency for the housing to leak. This system is also very simple, because the controls of the equipment inside the housing can mostly be operated through the flexible plastic. There are limitations to this approach however, because as you go deeper, there will come a point when the housing clings tightly to whatever is inside it, the housing won't be able to shrink anymore, and a pressure difference will begin to appear. Consequently, flexible housings still need to have a pressure-resistant sealing system. The ewa-marine housing shown above uses a metal clamp, held together by hand-tightening screw-fasteners ('hand-wheel nuts'). Closing the housing is a matter of making sure that the two surfaces that will be clamped together are clean, since dirt, and particularly hairs, can make microscopic channels through which the water can creep. Cleaning the surfaces is simply a matter of wiping them with a lint-free cloth or paper towel, and inspecting them carefully before fitting the clamp. When assembled properly, the sealing system, and the tough flexible plastic material, can withstand large pressures, 30 or 40 metres of water, and the real limitation is that of how much punishment the thing inside the housing can take. With cameras, there comes a point at which the water starts to press buttons that you don't want pressed, and so the recommended depth limit for flexible housings is usually (but not always) about 10 metres.
     Flexible housings are available for cameras, video cameras, personal effects and valuables (so you can take them with you into the water), mobile phones and VHF radios (in case you fall into the water), maps and charts, etc.

Hard, Rigid, or Pressure-Resistant Housings
Hard housings, whether part of a camera, or a separate box into which the camera must be installed, are nearly always sealed by means of a device called an O-ring. In fact, there will be an O-ring seal for every control, and every plug, socket, or window, in the housing, but the user is mainly concerned with O-rings (such as the one around the door that gives access to the camera) that have to be disturbed in some way while the equipment is being prepared for use.

DC5 open

The O-ring itself is simply a rubber band, usually of circular cross-section, but manufactured to high tolerances so that it is free from nicks, holes, and other defects that might let the water into the housing. The O-ring is also made of a particular type of rubber (there are lots of types), chosen for its chemical resistance and hardness, and it is not a good idea to use a substitute unless you know what you are doing.


The 'O-ring seal' is the combination of the rubber ring and a specially shaped groove, which is designed so that, when the housing is placed under pressure, the ring is forced into the gap through which water is trying to pass.

O-ring with no pressure difference
O-ring under pressure

 The cute thing about this system is that the seal actually gets tighter as the pressure increases; which means that the seal is inherently reliable, provided that the O-ring and the O-ring groove are kept in good condition and are kept thoroughly clean while the housing is being closed. This is the most important thing you need to know about underwater photography with hard housings, because bad husbandry in this department can lead to serious disappointment, whereas bad photographic technique can be rectified with practice. There is nothing difficult about assembling an O-ring seal however; it is simply a matter of cleaning the O-ring and the groove with a paper towel, inspecting the surfaces for dirt and hairs, and, for the type of seal shown above, applying a very thin coating of grease to the O-ring. The grease used on O-rings incidentally, does not form part of the seal; its job is simply to make the O-ring slippery so that it can slide into position without being damaged.
     Most manufacturers opt for the hard-housing approach to underwater equipment design, and a wide range of camera housings, lights, flash units, and torches are available, with depth ratings from a few metres to over 100 metres. You can find out more about underwater photography by reading the other articles in this website, and by looking-up terms you don't understand in the glossary.

Going underwater
There are a few people who decide that they would like to take underwater pictures but have, up until that point, always kept their eyes tight shut when going under the surface. It is important to know therefore, that the human eye cannot focus underwater. In order to be able to see the camera viewfinder, it is necessary to place an air-space between the eye and the water. A pair of swimming goggles will do the trick at shallow depths; but these can cause injury at depths of only a few metres because they lack any means of pressure equalisation.  Plastic goggles also tend to become scratched and battered, and you will be able to see much more clearly if you obtain a proper diving mask with toughened optical-glass lenses. You can obtain such a mask from any diving equipment supplier, but you should also seek advice on choosing the right size and proper fitting. Diving mask
     Masks and goggles also tend to mist up, and clarity of vision is much more important for photographic diving than for any other type of diving. A solution to this problem is to apply the tiniest amount of shampoo to the inside of the lens, and wash it around with a little water. Since traces of the detergent will inevitably end-up in your eyes, you must use a shampoo that doesn't cause the eyes to sting for this purpose, i.e., baby shampoo (based on sodium lauryl ether sulphate SLES), or the type of shampoo for which you have to pay extra to have the noxious chemicals left out. Don't use medicated shampoo, conditioning (i.e., oily) shampoo, and don't be tempted to use washing-up (dish) detergent (SLS) or ordinary soap. If you have no shampoo with you, saliva (spit) is the traditional diver's de-fogging agent, but it is not as effective as detergent.
     You can take perfectly good photographs by putting your head just under the water; but to add a feeling of drama to a photograph, it is sometimes desirable to be on the same level as, or below, the subject. This means that you will need to learn how to dive or snorkel, and this brings us back to the subject of ears. One of the golden rules about swimming under the water is that there is no acceptable level of pain. Your Eustacian tubes will not work very efficiently if you have a cold; and people often have low-grade throat infections that produce little in the way of symptoms except for difficulty in opening the Eustacian tubes. The Eustacian tubes of children also tend to be inefficient, but these problems usually resolve naturally by the time that children reach their teens. If your Eustacian tubes do not open as you dive down, you will experience discomfort at a depth of about 0.5 metres, and actual pain at a depth of about 1 metre. The only way to resolve this problem is to go back to the surface and try again. If you carry on down, the pain will not go away because the pressure difference between the middle-ear and outside will hold the Eustacian tube shut, and you will eventually reach a point at which one of your ear-drums will burst. verboten Don't do it - learn how to get your Eustacian tubes to work instead.
     Opening of the Eustacian tubes is normally a part of the yawning and swallowing reflexes, which is why it's a good idea to feed babies while an aircraft is coming in to land. You can actually yawn without opening your mouth, and you might try learning how to do this (out of the water) and using this trick when you dive. When the Eustacian tube to a particular ear opens, you will hear a 'click' in that ear. If you hum a note at the same time, the sound of the humming will become loud as the air-channel between the ear and the throat opens. With practice, you might find that you can get the opening of the tubes under conscious control, and separate it completely from the yawning reflex; after which, diving will become a cinch. If all else fails however, you can resort to a procedure known as the 'Valsalva manoeuvre', which involves holding your nose and blowing (you must also close your mouth or block your snorkel with your tongue). Diving masks normally have provision for holding the nose from the outside, so that the Valsalva manoeuvre can be performed while wearing a mask; but it is not a good idea to perform this procedure too vigorously if you have a cold, because you may introduce bacteria into your middle-ear and cause a painful infection (and temporary deafness). The Valsalva manoevere should anyway, only be performed gently and tentatively, with just enough pressure to achieve the desired effect. Coming up from depth incidentally, is hardly ever a problem, because the closed Eustacian tube acts as a one-way valve; one of its natural rôles being to allow fluid to drain easily out of the middle-ear, but make it difficult for things to get in.

dot_star dot_star dot_star

You can find out more about underwater swimming and diving by contacting training organisations and clubs in your local area. If you are planning the diving holiday of a lifetime, you will find that you get the most pleasure from it by taking a training course at home before you go.
     You can find out more about underwater photography by reading the other articles in this website. If you don't know what a particular technical term or abbreviation means, there is a good chance that it will have been added to the glossary.

© David W Knight. 2001, 2002, 2012, 2018, 2019.

Optics & Photography