Page 242 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 8: Field Photography 221
In the mid-1960s, Nikon released the revolutionary 28-mm water- corrected lens. For the first time, there was an off-the-shelf, custom-designed, water-corrected lens. The improved quality of the photographs was imme- diately obvious. It should be noted that Nikon rated the focal length of their underwater lenses in air (even though they cannot be used in air), so that the effective focal length of the 28-mm lens in water is 36 mm. In the early 1970s, Nikon brought out the 15-mm lens (20mm in water) and, until recently, this was the single most effective lens available for maritime archaeological work. The lens is virtually distortion-free; certainly, there is almost no spherical distortion at all, and recent measurements indicate that, for all practical purposes, it can be considered a stereometric lens. It rep- resents an impressive technological feat enabling photogrammetric mea- surements to be made using this system. The short focal length reduces the camera-to-subject distance, thus reducing attenuation of the subject in bad visibility. In clear water, close-ups look as if they have been taken in air. The lens was redesigned for the Nikonos IVA and V cameras and in its present form will fit any Nikonos camera except the RS model.
The 15-mm lens comes with a large, clip-on viewfinder which is easy to operate under water, though some care is necessary to take account of par- allax when working close up. The lens has an excellent depth of field, and it is a tribute to Nikon that they did not take a short-cut and make the lens fixed focus. At high light levels using f 16, the depth of field is from infinity to 0.25 m so that in such conditions, one can virtually use it as a fixed focus lens. At f 2.8 the depth of field is still good, infinity to 1.5 m, 1.0–0.6 m, or 0.32–0.27m close up. Figure 8.1 shows some of the range of the Nikonos system. Nikon also added a 20-mm (27 mm in water) lens to the range.
Additionally, a new, non-Nikon, and consequently much cheaper lens, has been brought out by Sea and Sea. The initial reports are that the lens is of high quality, and that it may prove to be a useful alternative to the 15-mm Nikon lens.
III. MISCELLANEOUS EQUIPMENT A. EXPOSURE METERS
Another accessory for underwater photographic work is an exposure meter. Light levels under water are quite difficult to judge, particularly as our eyes accommodate to different light levels. An accurate exposure meter is very useful when working in natural light with the nonautomatic cameras like the Nikonos I and III. A battery-powered cadmium sulfide cell expo- sure meter is a good reliable choice.