Flat or Dome Lens Ports
Shooting through a flat glass port underwater causes several distortions to the image. For this reason, generally speaking, flat ports are reserved for out of the water shots, split level shots and extreme closeups underwater. It’s important to know what’s happening optically with flat ports.
When light rays pass through media of different optical densities, in this case, water, glass and the air inside the housing, they bend at different angles. This is called refraction. It is this unequal bending of light rays that causes all various distortions when shooting through flat glass. The most obvious effect is magnification. Just as when you look through a dive mask everything you see is magnified, so is the image for a camera, so the effective focal length of a lens is increased by approximately 25%. As a result, with a flat port, lens focus must be set to approximately 25% closer than the actual sensor to subject distance. 25% is not an exact number so always rely on eye focus rather than doing a calculation.
More critical to the optical quality of your image are the following distortions caused when shooting through flat glass:
Radial Distortion: This is what we see commonly with fisheye lenses and when straight lines appear to be curved in the image. This can show up either as barrel distortion or pincushion distortion and the flat port can cause this effect.
Chromatic Aberration: White light, when refracted, is separated into the color spectrum. The component colors of white light do not travel at the same speed, and light rays passing from water to glass to air will be unequally bent. When light separates into its component colors, the different colors slightly overlap, causing a loss of sharpness and color saturation.
Light rays passing through the center of the flat glass into the center of the lens travel in a straight line and are therefore not bent in different angles. But as the rays pass through the port farther toward the edges, the rays bend in increasingly greater angles. As a result, all of these distortions are more apparent with wider angle lenses and when viewing subject matter closer to the edges of the frame.
The dome port is a precision ground lens with concentric inside and outside surfaces which, when installed on an underwater housing, acts as an additional optical element to the camera lens. When light rays pass through the concentric surfaces of a dome port, just as through the center of flat glass, they pass in a straight line, but with the dome they do so throughout the whole area of the port. Therefore, a dome port eliminates the refraction of light and the subsequent magnification, radial distortion and chromatic aberration caused by flat ports. Because the magnification is eliminated, the lens retains its normal focal length and angle of view.
Because the dome port acts as an additional optical element it shifts the focus of the lens and creates what is called a “virtual image” in front of the lens much closer than the actual subject distance. To photograph a subject underwater with a dome port you must focus the lens on the “virtual image,” not the subject itself. The dome port makes the footage marks on the lens totally inaccurate for underwater focus. Therefore, lenses should be calibrated underwater. Out of the water, if the lens is properly placed in the housing, the dome port offers no special optics and functions as a clear window with normal focusing properties.