When you are shooting photographs underwater, your will encounter different obstacles in lighting your ideal image than you would in air. Water refracts light at a higher index than air, which makes objects appear larger and closer than they are. Water also absorbs different wavelengths of light depending on depth. This means that the longest and weakest wavelengths of color, such as red and orange, are absorbed before blues, greens and violets. Water will even dim the contrast and sharpness of your photos.

The most important factor for optimum light for your underwater subject is the quality of light. The light that creates your camera's exposure will come from the sun and optionally camera strobes and flash< /a>. Quality of light depends on four different factors, regardless of what the light source is: 1) The amount of light visible 2) the color of the light, 3) the direction of the light, and 4) the amount of diffusion or softness.

Visibility in water decreases exponentially the deeper you go from the surface and the farther you are from your shot. Even in clear water, the recommended distance to your subject is 12 inches or closer for a macro or close up photograph. For distance shots, preferably in good weather and clear water, a maximum distance of 40 feet (12 meters) is a good guideline.

Even in the most tropical, clear blue water you will need to bring lights like a camera flash or underwater strobes with you. When shooting in natural light, it can be difficult to get the higher shutter speeds necessary for very sharp photos, unless they are shot in bright light at large apertures. This can work for you if your goal is a backlit silhouette shot or for creative effects such as panning or showing motion. For everything else, use your strobe lights and your flash. A strobe or flash provides filler light that heightens your shutter speed. The full spectrum of visible light you now have will bring back the missing reds and oranges that are so easily lost at depth, along with clarity and detail.

Always review the weather for the day of your photo shoot, to ensure you have strong sunlight and calm water. The more disruption from water currents and cloud cover you get, the more difficult it will be to get that clear, sharp picture. Be aware of any silt, sand or debris that may be floating in your area, and always watch for fellow swimmers and worsening weather. Backscatter, those blurry grey dots that can ruin an otherwise perfect picture, worsens when the level of floating particulates is high.

The last factor to consider is whether or not the ambient or artificial light in your scene is “hard” or “soft”. Hard light has a bright spotlight quality that creates higher contrast and sharp shadows. Soft light is diffuse and usually comes from multiple directions, creating less contrast. Choosing a diffuser for your camera can soften the effect of strobes, spread out the light for wide angle shots, and reduce the backscatter effect. Whether or not to use a diffuser depends on the photographer's situation and camera subject. So make sure you bring the right equipment for your purposes.