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Lens choice & light refraction

Wide-angle lenses can look normal underwater.


Objects underwater look closer than they actually are. Photographer Tom Sheldon caught this hermit crab sifting through the soil for food.
Objects underwater look closer than they actually are. Photographer Tom Sheldon caught this hermit crab sifting through the soil for food.

WATER ACTS LIKE A MAGNIFYING LENS

Underwater subjects look closer than they actually are due to refraction (bending) of light by water.

Refraction is a change of direction of a ray of light. Light bends when it passes through light-transmitting substances like water and glass at any angle except a 90 angle. The density of water, about 800 times denser than air, causes a change in the speed of the light, making it alter course to travel through the water. As a result, when you look at subjects underwater, they appear closer than they would at the same distance above the water. In fact, they look about one-fourth closer, which means a subject that is 16-feet away seems to be 12-feet away. If you were to hold a magnifying glass that increases size by 25% in front of you now and focus it on an object, that is how the object would look to the naked eye at the same distance underwater.

Cameras with through-the-lens focusing give the underwater photographer a distinct edge. Photo by Tom Sheldon.
Cameras with through-the-lens focusing give the underwater photographer a distinct edge. Photo by Tom Sheldon.

LENS CHOICE

Water’s refractive qualities affect your camera’s lens in the same way they affect your vision. Because of this magnifying effect of the refractive index of water, a wide-angle lens, rather than a normal or telephoto lens, is preferred for taking pictures underwater.

In fact, the effective focal length of a lens is increased due to water’s magnifying properties. It’s like holding a magnifying glass in front of the lens. A 35 mm lens underwater has roughly the same angle of view as a 50 mm lens on land.

There are other advantages to using a wide-angle lens underwater. With a wide-angle lens, you must move closer to your subject to fill the frame. This results in there being less “thickness” of water between your lens and your subject, therefore contributing to making your pictures clearer. Lenses of 35 mm, 24 mm and 21 mm can be used effectively underwater. Many undersea photographers especially prize lenses that are 18 mm, 16 mm, 15 mm and even 14 mm, which have less of an extreme wide-angle-distortion effect underwater than they do on land.

The greater depth-of-field of a wide-angle lens gives it another advantage, since you will most often be shooting with a wide aperture due to low light levels underwater. Although finding a firm place to lodge your camera, such as wedging it between two rocks, while still having the lens properly aimed at the subject and at the right distance will permit you to use smaller apertures and slower shutter speeds.

If you will be shooting small creatures and other tiny subjects, a macro lens can be used underwater. 100 mm and 105 mm macro lenses are ideal, providing great magnification of the tiniest of creatures.

It is a good idea to stick with one lens until you have mastered its use underwater before switching to a different lens. Of course, you can't change lenses underwater, so you must decide which lens to use before you dive. Your decision will be influenced by water clarity and the size of your subject.

The water's clarity influences the type of lens you can use.
The water's clarity influences the type of lens you can use.

If the water is murky, you will want to have as little of it as possible between your subject and your camera, so a macro lens or a close-focusing lens may be your best choice. If, on the other hand, the water is clear, but your subject is large (an undersea panorama, a sunken wreck or a huge animal), then a wide-angle lens would be your best choice.

A medium-range lens is suitable for general underwater photography, from small objects to a portrait of a fellow-diver, particularly if it has good close-up focusing capabilities, but if you follow the lead of most accomplished underwater photographers, a good wide-angle lens will probably be your most-used lens.

This remarkably good image was taken with, believe it or not, a disposable camera. Photo courtesy of photographer Dick Dougall, Brentwood Camera Club inTennessee. Click on the image for more of Dick's amazing disposable camera pics.
This remarkably good image was taken with, believe it or not, a disposable camera. Photo courtesy of photographer Dick Dougall, Brentwood Camera Club inTennessee. Click on the image for more of Dick's amazing disposable camera pics.

PORTS

Many camera housing units come with "domed ports" (a half-sphere-shaped window in front of the lens) that correct for refraction so that the lens' angle of coverage is the same as on land. Flat ports are also available, but they are intended mainly for close-up macrophotography, using a macro lens. Domed (rounded) ports are used with wide-angle lenses for landscape-type images and other wide-angle applications. A good dome port is of high optical quality.

The most-common problem occurs when the lens is too close to the dome, resulting in a loss of sharpness from refraction. The lens must be positioned such that light entering the port strikes the lens at 90 angles.

SETTING FOCUSING DISTANCE

If you are able, with your camera and its waterproof housing, to focus by looking through the lens underwater, then you can fill the frame without concern for the difference between apparent and actual distance caused by refraction. You may also have a special underwater camera that permits through-the-lens focusing. If not, the distance scale on your lens should be set for three-quarters of the actual distance.

This means that for a subject at 8-feet, the distance scale should be set for 6-feet to obtain sharp focus. Since you will probably not be carrying a measuring tape with you, and since both lens and eye are affected equally by refraction, you can focus the lens by estimating subject distance by eye. If it looks to be 3-feet away (even though it is actually 4-feet away), then set the lens’ focusing scale for 3-feet.

 
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