In underwater photography, the use of a water-proof cylindrical port is necessary to accommodate the lens of a still camera. A port can be attached directly to the water-proof housing or to an extension ring before attachment to the housing, depending on the length of the lens. Very often, manufacturers specify which lenses a particular port is suitable for.
The refractive property of the water leads to edge distortions of images, this effect is especially significant when using wide angle lenses and fish-eye lenses. A dome-shaped port can be used to eliminate or at least reduce the distortions.
On the other hand, the effects of refraction are desirable when using a standard or a macro lens; these include the reduction of the angle-of-view and the increase of the magnification of the lens. Therefore, a flat port is normally used when shooting small objects underwater.
Different light sources have different spectral properties, meaning that they emit light at different colour temperatures. A light source which has a low colour temperature implies the light it emits is shifted towards the warmer end, giving the image a red/orange cast; on the contrary, a high colour temperature implies the light is shifted towards the colder end, giving the image a blue cast.
It is therefore of significant importance for a photographer to take this phenomenon into account in order to produce colour-balanced, natural-looking images.
Most digital cameras have an auto white balance function and some preset white balance settings, such as tungsten, fluorescent, cloudy, and sunny, however, these are not always consistent and accurate as they are not based on the exact colour temperature at the instant of shooting. Therefore, many cameras have a built-in custom white balance function which allows the photographer to calibrate the white balance according to the lighting environment at that moment.
One method of achieving an accurate white balance requires the photographer to take a photo of a calibration target, which has to be a white or grey object; the camera then uses this as a reference to set the white balance. At present, white and grey cards are available in the market for this purpose. Alternatively, a translucent white filter can be placed in front of the lens and by capturing the incident light; the correct white balance can be obtained.
White balance has to be re-set when the lighting environment changes, this implies the photographer has to look for a white/grey object, use the white/grey cards, or attach then detach a translucent white filter every time the lighting situation changes, which when shooting on land, can be done relatively easily.
In underwater photography, achieving an accurate white balance is considerably important, as a result of the ability of the water to absorb light of long wavelengths, such as the red and orange, causing objects to appear blue-green. The effect becomes more significant with increasing water depths. This demands that the photographer frequently calibrates the white balance underwater at varying depths.
Similar methods as the ones mentioned above for shooting on land can be adopted for customising white balance underwater; however, the utilisation of a port leads to the impossibility of repeatedly putting on and taking off the calibration device from the lens. Hence, the calibration device has to be attached outside the port and taken off after the white balance is set. Instead, an underwater photographer can make use of white/grey cards; both methods imply the need for the photographer to carry extra objects underwater, this not only inconvenient but also increases the water resistance of the photographer. Some underwater photographers therefore choose to make use of white or grey objects underwater as calibration targets, but again, this method cannot guarantee accurate white balance settings.
Thus it can be seen that setting white balance while underwater presents considerable difficulty.