The art of taking photographs underwater is practiced by scuba divers, snorkelers, and swimmers, as well as researchers for the purpose of capturing images of such wonders as marine life, sea caves, and shipwrecks. However, water is a different medium than air and it presents its own set of unique challenges. This article examines one particular problem of underwater photography – what is refraction, and which solutions are there to counteract it.
Refraction, as it pertains to photography refers to the bending of light rays when they travel at oblique angles from the air in the dome port of the photographer’s camera, or mask, to the water. When this occurs, it results in flat, straight edges having a rounded appearance around their perimeter. It most often happens when using a lens with a wide angle and a flat port.
The refractive index of water is around 1.33, whereas that of air is much smaller at 1.0003. It essentially functions like a magnifying glass, making objects appear about 25 percent larger and closer underwater. This can be easily demonstrated by placing half of a pencil in a glass of water, and taking note of how the pencil appears bent at the water’s surface; this is all the result of the light rays bending where they transition from water to air.
As a rule, the image displayed on the LCD screen or viewfinder accurately depicts how the photograph will look. Housing port design, and the exact location where the lens focuses can also make a difference. Manufacturers are aware of this occurrence, and have designed ports to work around this limitation to an extent, but it is sometimes necessary to use a diopter for optimal underwater focus.
When a lens is used behind a flat port, this alters its angle. One which is 35 mm has the effect of a 50 mm lens. The best solution to this dilemma is to use a correction port or dome that will direct all light rays moving from air to water so they travel at right angles, which effectively eliminates the effect of refraction. Some other, more complex correctors can also restore the angle of the lens.
Using a camera which is contained within a watertight housing can be expected to cause image distortion, especially if a wide-angle lens is used. A fish-eye or dome-shaped port can fix this distortion in most cases. Such housing ports are available for most cameras, and many of them have been designed for use with specific lenses which will further increase their effectiveness.
Besides the dome port, the photographer can also use a wet-coupled lens. This supplementary lens is affixed to the outside of the lens port for the purpose of increasing the field of view and permitting simultaneous wide-angle and macro photography. A macro lens does not present the same problems with refraction as others do, so it can be used with a flat glass port, and it can actually work to the photographer’s advantage if increased magnification is desired.
The time of day one is shooting at can also play a role in determining how much refraction will occur. Midday is recommended as the most suitable time to shoot images, because the sun is directly overhead, which means that more light rays will hit the water at right angles and will not deflect as much, this results in less refraction.