Although serious underwater photography at a reasonable depth and for prolonged periods still remains a specialized field (and needs scuba-diving techniques), shallow waters are accessible to most people. The range of camera equipment available includes not only housings for standard SLRs, but amphibious models of varying degrees of sophistication. Many of the exposure uncertainties that once severely restricted casual underwater photography have now been removed by automation. This, together with the new popularity of underwater tourism, makes this a rewarding field.
Equipment
There is a wide range of equipment available, but the basic choice is between submersible housings for standard cameras and amphibious cameras that are themselves sealed against water at pressure. (Show Photo) Submersible housings have certain advantages. One is that, although the initial expense can be high for a metal housing, plastic housings are quite moderately priced and there is no need to buy extra camera equipment; the existing camera and its range of lenses can be used. Indeed, the range of focal lengths is another advantage, as is the ability to use an automatic camera's existing functions, including exposure metering and film-winder. Against this, housings are necessarily bulky, and if the optics of the port (in front of the camera lens) are not the best, image quality suffers. Also, careless assembly can cause leaks that could effectively write off the camera.
Amphibious cameras, of which the Nikonos has been the pioneer and prime example, have the advantages of being compact and purpose-built. The special optical problems of working underwater give a theoretical advantage to lenses that are computed for submerged use, and amphibious camera controls are deliberately simplified - ease of use is an important consideration when shooting underwater, where so much of a photographer's attention is taken up with diving. The Nikonos accepts a range of water-contact lenses of different focal lengths. The disadvantages of amphibious cameras are the additional expense and the non-reflex viewing, which causes complications with framing until the photographer is experienced.
Flash is essential for virtually all underwater shooting, as the section on optics below explains. There is a range of automatic and semi-automatic underwater flash units available, most mounted on jointed arms so that they can be fired from at least a short distance away from the camera to overcome back-scatter, the reflections from particles suspended in the water.
Underwater optics
Just as glass has a higher refractive index than air, so does water. Water has a pronounced optical effect. A stick dipped into water at an angle demonstrates this - it appears bent and foreshortened in the water. In effect, the water behaves as an extra lens in underwater photography, the most important results being that objects appear to be one quarter of the distance closer than they really are; objects appear to be one third larger than they really are; and the angle covered by any lens is one quarter less than it would be in air. (Show Photo)
These effects are interrelated and when shooting, the most obvious is probably the last one; a 35mm lens, for instance, which normally has an angle of coverage of 62 deg., covers only 46 deg. underwater - equal to the coverage of a normal, 50mm lens. In practical terms, what was a wide-angle lens becomes significantly less so. Naturally then, as it covers a smaller angle, objects fill more of the frame and seem closer.
Amphibious cameras and their lenses are built to take care of this, the controls and viewfinder being calibrated to take account of the refraction. With a standard camera in a housing, however, these optical effects are very noticeable. The design of the housing's port makes an important difference. A flat port does nothing to correct the refraction and, in addition, tends to exaggerate various optical faults: chromatic aberration increases, resolution falls off towards the edges of the picture frame, and pin-cushion distortion occurs.
The solution is a curved or dome port. This acts as an extra-large negative lens, correcting the refraction so that the lens angle of coverage and, therefore, the apparent size of the subject, is restored to what it was in air. In doing this, however, it introduces a new effect: the object being photographed appears even closer. In order to be able to focus properly, particularly on distant objects, most lenses need to have either a plus-diopter supplementary lens or a thin extension ring attached. Dome ports also correct some of the chromatic aberration, loss of sharpness towards the edges, and pin- cushion distortion.
Water also absorbs much of the light entering it. This is partly due to the surface of the water reflecting sunlight back into the sky (particularly in rough seas), and partly to particles in the water scattering light. Not only does water absorb light, it does so selectively, by wavelength. The first wavelengths to be absorbed, at even shallow depths, are at the red end of the spectrum. Lower down, yellow is absorbed, then green, leaving only blue at great depths. All of this, less light and loss of color, limits natural-light photography underwater. Color compensating filters can correct the color absorption only to about 10m (30ft) conveniently. In practice, the only way to restore color to underwater scenes is to light them and, for this reason, flash units are essential.
Flash techniques
Electronic flash is relatively straightforward to use underwater, provided that the flash is positioned away from the camera. The reason for this is back-scatter, which is the snowstorm effect caused by reflections off particles; it occurs when the light comes from close to the lens axis. The usual system is for the flash to be mounted on an arm and aimed from above and to one side of the camera. The advantage of this is that the photographer quickly becomes familiar with the lighting direction. Alternatively, the flash can be held at a greater distance on a coiled extension cable, preferably by a diving companion. An additional flash, triggered in sync by a photo-cell, gives more elaborate lighting possibilities. For close-up work, reposition the flash-head.
Natural light
Photographing by natural light is quite possible at shallow depths and is the only way of shooting underwater landscapes. The best weather conditions are bright sun, clear sky and calm, clear water. A wide-angle lens has the advantage of giving an apparently clearer image than does a longer focal length lens, as even the clearest water has large numbers of particles that degrade the view. In principle, one of the most effective types of composition is to shoot quite close to an interesting foreground, such as coral, and include some of the underside of the water surface. This gives both depth and contrast. A wide-angle lens is also less susceptible than a normal lens to camera shake, which can easily be caused by water currents and a slight negative or positive buoyancy - one hand is sometimes needed to hold on to coral or rock.
Even so, the light levels reduce the possible depth of field and call for slow shutter speeds, while a color compensating filter is needed according to the particular depth. In many ways, underwater landscapes by natural light are best combined with flash for the foreground. The foreground colors are then bright and normal, and their contrast, with the blue cast of the distance, is both acceptable and attractive (use no color compensating filters with flash).
