Optical problems in underwater photography are primarily due to the fact that 

 in most cases the cameras used are designed for operation in air. To operate 

 under water, the air-camera must be housed in a water-tight container. There 

 obviously must be a "window" or port in front of the lens. This port, in most 

 cases, is a flat piece of glass or plexiglass. Herein is the optical problem. 

 Due to the laws of refraction, light passing through a flat or plane "window" 

 makes objects look closer than they actually are. This apparent object appears 

 l/4th closer than the real object being photographed. "The field of view of 

 the camera will be reduced by approximately 1/1.33 as a result of refraction at 

 the port. This means that a wide-angle lens will be converted to a normal lens 

 and a normal lens will be converted to a mild telephoto lens by the magnifying 

 characteristics of the port. In-water applications almost always require the 

 widest fields of view, and this reduction can be quite detrimental."* It is 

 necessary to get close to the subject being photographed underwater for two 

 major reasons: 1) Any sediment, or other matter in suspension, causes a lack of 

 sharpness of the image in the resulting photograph. 2) The more water between 

 the camera and the subject, the more degraded will be the saturation of the 

 color of the object being photographed due to the water acting as a blue filter. 

 Therefore it is of prime importance that an UW photographer be able to utilize 

 his wide-angle lenses at their full focal length. Besides the magnifying 

 effect, the flat-port causes other aberrations that degrade the image, one of 

 the most important being chromatic abberation, especially bothersome to the UW 

 color photographer. 



Flat-ports also cause serious "pin cushion" distortion of straight lines with 

 wide-angle lenses. The flat-port also has serious out-of-focus areas at the 

 outer edges of the image. For a complete scientific discussion of the physical- 

 optical aspects of underwater photography, see G. T. McNeil in Optical Funda- 

 mentals of Underwater Photography, Photogrammetry, Inc., Rockville, Md. (1968). 

 It is not our objective to set forward here the scientific aspects of UW optics. 

 Our approach has been entirely empirical. 



The Dome-Corrector 



Various solutions or partial solutions to some of the aberrations of underwater 

 optics have been effected by the Ivanof f-Rebikof f correcting lens. This system 

 comprises a reversed Galilean telescope in which the front concave lens is used 

 for the watertight window. The UW Nikkor 28mm lens (which we included in our 

 test) and the UW lenses designed by E. Leitz of Canada, known as Elcan lenses, 

 are examples of such solutions. These systems are quite costly, and since the 

 dome-corrector is quite inexpensive, we felt that what was needed were practical 

 tests of the dome-corrector system versus the flat-port system on a direct lens- 

 by-lens comparison basis, photographing an underwater test pattern, so that edge 

 sharpness and other linear distortions could be compared. We also planned on 

 the testing of various diameter spherical dome-ports to learn how they changed 

 camera-lens focus as opposed to real UW camera subject focus distance. We had 

 learned by trial and error methods that there were distinct and drastic differ- 

 ences. In other words, we found that when using dome-ports, magnification 

 was almost entirely absent. When, for example, an UW subject was 10 feet 



* Mertens, Lawrence E., "In-Water Photography," John Wiley and Sons, New York, 

 p. 146. 



VI-296 



