Lens Angle Underwater

Most popularly used cameras, however, are designed as a land camera with a fixed focal length of 38 mm or 45 mm somewhat on the wide-angle side. Even cameras with a zoom capability are usually designed as a land camera with the field angle (or the light-receiving angle) of the lens selected to cover a portion of the so-called wide-angle region and a portion also of the so-called telephoto region such as the range of 35-70 mm. When such a camera is used in water, it cannot function as a wide-angle camera. In other words, since the index of refraction of water is 1.33 and considerably higher than that of air (=1.00), a lens with a fixed focal length of 35 mm or 45 mm in the above-water condition (or in air) will function in water as a lens with a focal length of 50 mm or 65 mm, and a zoom lens of focal length 35-70 mm in air will function as a zoom lens of focal length about 50-100 mm in water. In either case, the lens will not function as a wide-angle lens.

Here's a table showing typical angle of view for lenses on different camera systems...





Macro photography under water
The secret is to work with a small strobe, placed back sufficiently to still appear large (some need a diffuser), while lighting nearest and furthest point in the focal plane with the least difference. By pointing the light a little too far to the opposing side, one uses the environment as reflector to bring light in the shadows while also lighting less of the near side. Understandably, when shooting at variable distances, this compromise must be rearranged each time, resulting in constant adjustment of the strobe. How easy you can do this, makes a large difference to your chance of success.