Page 252 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 8: Field Photography 231
will be. Thus the system has to be tested a number of times before it is certain that it is working and, should the conditions change, the arrange- ment will have to be reconsidered and tried again. It is essential with all flash work to conduct pretesting of the system to ensure the system will provide proper coverage.
There are other problems that occur when working at low light levels. For example, a curious “pooling” effect occurs in these conditions with the 15-mm lens (Figure 8.4). It appears in the photographs as if there is a central bright spot surrounded by a dark area. This is probably caused by the long light paths that occur with wide-angle lenses at the edge of the frame and the relatively shorter paths close to the optic axis. There may be increased attenuation along these paths, because there is about a 50% increase for the path passing through the edge of the frame, in comparison with the path length of an axial path, to the object at the same distance from the lens.
In very low light conditions, time exposures can be utilized, provided that the camera can be held steady during the exposure. Unfortunately, the nonautomatic Nikonos cameras do not have measured shutter speeds below a 30th of a second, so exposures have to be timed manually. With the Nikonos IV and V, the automatic system allows for electronically controlled time exposures through the internal exposure meter (with the Nikonos V timing automatically goes up to 15 seconds).
Figure 8.4 Photograph showing the effect of “pooling,” where with low light levels and a very wide angle lens, there is a light center and a darker surround. (Courtesy of Patrick Baker, Department of Maritime Archaeology, Western Australian Maritime Museum.)