One of the most important and often misunderstood features of an underwater camera housing is the lens port. Flat ports were all that were available for underwater photography from its beginning in 1893 until 1931, when hemispherical dome ports were first used to correct for the refractive properties of water. Flat and dome ports both have their place in underwater photography and it is important to know the theory and practice of each. For underwater photography with wide lenses, the dome port is the best choice, but if the shot starts or ends above water, or if you need to shoot a close-up with a long lens, the flat port should be used.
Flat Port – The flat port is unable to correct for the distortion produced by the differences between the indexes of light refraction in air and water. Using a flat port introduces a number of aberrations when used underwater. They are:
Refraction – This is the bending of light waves as they pass through different mediums of optical density (the air inside the camera housing and the water outside the lens port). Light is refracted 25 percent, causing the lens to undergo the same magnification you would see through a facemask. The focal length of your lens also increases by approximately 25 percent.
Radial Distortion – Because flat ports do not distort light rays equally, they have a progressive radial distortion that becomes more obvious as wider lenses are used. The effect is a progressive blur, that increases with large apertures on wide lenses. Light rays passing through the center of the port are not affected because their direction of travel is at right angles to the water-air interface of the port.
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, which is more noticeable with wider lenses.
Dome Port – The dome port is a concentric lens that acts as an additional optical element to the camera lens. The dome port significantly reduces the problems of refraction, radial distortion and axial and chromatic aberrations when the curvature of the dome’s inside radius center is placed as close as possible to the nodal point of the lens. When a dome port is used, all the rays of light pass through unrefracted, which allows the “in-air” lens to retain its angle of view. Optically a “virtual image” is created inches in front of the lens. 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. The dome port offers no special optics above water and functions as a clear window.