Anyone who has put on a mask and snorkel and floated over a coral gardenor sunken boat will have had a glimpse of the fascinating world beneath the surface of the ocean. But we are not built to exist for long underwater and nor are most cameras. It is an alien environment with new rules for the photographer.

The nice thing about underwater photography is that you can approach it at any level. It is possible to take satisfactory pictures with an ordinary land camera through the 'window' of a glass-bottomed boat, or even in rock pools using a polarising filter, bucket or watertight box with a glass base. If the sun is shining on the subject, the pictures will be bright and clear.

But be careful when you are near water, especially salt water. Ordinary land cameras are like cats - they just don't want to get involved with water, and one drop of salt water in the wrong place could ruin your camera. So if you want to take a camera underwater, you will need either a purpose-built underwater camera or a watertight housing.

Kodak and Fuji now make a disposable waterproof 35mm camera with a built-in flash which can be used to a depth of two metres for very simple photographs. Canon produce the 35mm compact automatic Sureshot A1 with built-in flash which is waterproof to five metres, and Minolta make a fully automatic Vectis Weathermatic APS compact camera with built-in flash which is rated to ten metres depth. For deeper diving, Sea & Sea produce three models of 35mm underwater camera. The simplest is the MX5, a 'point and shoot' fully automatic camera rated to 36 metres. The MX10 is a basic 35mm automatic camera with built-in flash, but other lenses, which can be changed underwater, and a detachable flash are also available. The Motormarine 11 EX is a more sophisticated camera with TTL metering and a choice of dedicated flash accessories as well as a built-in flash. This too is a modular system, and lenses can be carried and changed underwater. Both these cameras are rated to 45 metres. Bonica also manufacture the Snapper, rated to 40 metres and another modular system that you can upgrade as your interest grows.

The great advantage of the modular system is that you can change from wide-angle to close-up lenses and vice versa underwater very quickly. Inevitably, when you have set up your camera at the surface with a particular subject in mind, something else will turn up on the dive which you cannot photograph with the lens on the camera. With camera systems that allow lenses to be changed underwater this is no longer a problem.

The Nikonos is probably the underwater camera most used by scuba divers. No larger than an ordinary 35mm camera, it is easy to operate and can give excellent results. Based on the French Calypso design, it is continually being improved. The Nikonos V has a fully automatic exposure system and a range of TTL automatic flash guns. There is no rangefinder for focusing, so you have to estimate focusing distances. As it is a non-reflex camera, with a direct vision viewfinder, you may have problems with parallax when close to your subject, but there is a compensation within the viewfinder to overcome this. An external sportsfinder frame can be fixed to the top of the camera to make viewing easier. The Nikonos V offers a choice of auto or manual exposure. An LCD display in the viewfinder tells you the shutter speed, warns of wrong exposure and provides a 'flash ready' signal.

The standard lens is the W-Nikkor 35mm, f2.5, and 15mm, 20mm, 28mm and 80mm lenses are also available. Special close-up lenses or extension tubes allow detailed shots of coral and tame fish.

Underwater housings

Rather than investing in a whole new camera system, an alternative approach is to use an underwater housing around your land camera. In shallow water of less than ten metres, flexible plastic housings provide a relatively cheap method of protecting your camera. Controls are operated through a rubber glove set into the case.

In deeper water the flexible design is unsuitable, as the housing collapses under increasing pressure. Ikelite housings can be made for most cameras. These are rigid and some models can safely be taken to a depth of 100 metres. They have controls which link into the focusing and aperture rings, as well as shutter release and film advance mechanisms.

Rubber 'O' ring seals produce a watertight chamber which keeps the camera dry. To avoid flooding, the rings must be cleaned and lightly greased with silicone each time a film is changed. A range of aluminium alloy housings is also readily available; the housings are strong and durable, but heavy and bulky. Plexiglass housings are lighter and cheaper, and are available for a wide range of cameras.

Most serious photographers will already have a camera and range of lenses, so a cheaper option may be to buy a ready-made housing which will tend to be more versatile. Most camera features such as autofocus and motordrive can be used underwater. A comprehensive range of housings is available off the shelf from many manufacturers, who can also produce individual housings for unusual cameras without any difficulty. Housings and lights are also readily available for many types and makes of video cameras, including the new generation of compact digital video cameras.

How light behaves

Light is refracted (or bent) more in water than in air. Underwater objects appear larger and nearer than they really are. Your eye perceives the same distortion as the lens, so with a reflex camera you simply focus through the lens and the subsequent picture will then be in focus. The subject may be 1.5 metres away, but will appear closer to the eye and to the lens. However, if you then look at the focusing ring, it will be set at about one metre.

Because of the way light refracts through water, the effective focal length of the lens is increased, making it more telephoto when a flat underwater porthole is used. So a 35mm lens underwater is approximately equivalent in effect to a 45mm lens on land. Likewise a 15mm lens is equivalent to a 20mm.

A dome-shaped porthole, on the other hand, enables light from all directions to pass through it at right angles. This eliminates the problem of refraction and the angle of view of the fitted lens is unchanged.

Lenses

Wide-angle lenses are generally more useful underwater. Visibility is seldom as good as above water, especially if there are numerous suspended particles. For a clear image it is important to move in close so as to reduce the amount of water between the camera and the subject. To include the whole of a diver in the frame when using a 35mm lens on a Nikonos, you need to be about two metres away. A wider lens, say 15mm, means you can move in much closer to the subject and thus minimise the amount of obstructing material between the camera and the subject.

Generally camera-to-subject distance should not exceed a quarter of the visibility. If the visibility is only 1.5 metres (as it often is in temperate seas or inland lakes), you should restrict yourself to subjects up to 0.3 metres from the lens.

Flash

With high-speed emulsions such as Ektachrome 400 (transparencies) and Kodacolor 400 (prints) it is often possible to get away without using flash, especially near the surface where it is brighter. When the sun is shining through the surface layers of water, you can obtain good results down to about two metres without flash. However, the deeper you go below the surface layers of water, the more the light is filtered out by the water. At ten metres below the surface, the red has been filtered out of the ambient light, and flash is needed to restore the absorbed colour.

In tropical waters, the guide number of the flash gun (which indicates its power) is usually reduced to about a third of the 'in air' number. It is much safer to bracket your exposures, as the expense of film is nothing compared to the trouble and expense of getting into the water.

Underwater flash guns are either custom-made or normal land units in a plastic housing. Custom-made guns generally have a good wide-angle performance, whereas units in housings generally have a narrow angle.

Instead of using a flash gun mounted close to the camera, place it at arm's length or even further away to give a better modelling light to the subject. Having two flash guns is even better, and will give much greater control over lighting. With the flash gun further from the camera, fewer particles between the camera and the subject will be illuminated. If the flash gun is near the camera, the particles will be illuminated and detract from the subject.

Aiming the flash can be tricky. Although your eye and the camera lens 'see' the subject as, say, two metres away, it is actually further. As the flash must strike the subject directly in order to light it, the unit must be aimed behind the apparent position of the subject. Some flash guns now have modelling lights so that you can see exactly where the flash is aiming.

Diving Problems

Test your equipment in a swimming pool before you take it into the sea. Plan the shots in advance. It is always better to have a good idea of what you want before you go into the water, so that you have the right lens on the camera to do the job.

Keeping yourself stable while trying to take a picture can be a problem. Underwater you should be slightly negatively buoyant, so that you can lie or kneel on the sea bed or hang suspended in the water without moving up and down. By breathing in you should make yourself rise slowly, and by breathing out you should sink. Wearing a stabiliser jacket will allow you to increase or decrease your buoyancy by letting air into or out of the jacket.

Sometimes you may need to grab a handy piece of coral to steady yourself. A wetsuit or thin Lycra one-piece suit will help protect you against stings and scratches. As you will be moving around slowly when taking pictures, you will feel the cold earlier than if you were swimming energetically, and you will appreciate the warmth the wetsuit gives you.

Near the sandy sea bed it is easy to churn up the water and disturb the sand, making the water cloudy. The secret is to keep as still as possible and use your fins gently. Restricting rapid movements also avoids scaring the more timid fish away. Taking a plastic bag of bread down with you usually guarantees plenty of potential subjects.

Good Subjects

Even with very simple equipment it is possible to record interesting effects simply by looking at what is naturally around you underwater. Rays of light burst through the water in a spectacular way, and are especially photogenic when they surround a silhouette. And you can get impressive effects by catching reflections on the surface when you look up at the sky through the water. Macro photography with an automatic camera and a frame placed over the subject is a good place to start, as it takes at least some variables out of the equation and makes success more likely.

The best pictures are usually simple and clear. Select a subject, such as an attractive piece of coral, then position yourself to show it off to best advantage without too many distractions in the picture.

Do not be deterred by a high initial failure rate: with a little thought and planning, achieving good results underwater is quite straightforward.

The following companies will be only too willing to talk to you or send you brochures, and their websites contain a wealth of information for both the beginner and the more advanced photographer:

Cameras Underwater: 01404 812277

Sea & Sea: 01803 663012

Ocean Optics: 020 7930 8408

Videoquip: 0116 2558818