Page 244 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed
P. 244

Chapter 8: Field Photography 223 and can dramatically improve the image. However, this is a complex process
and trial and error will be required to obtain the optimum configuration.
C. FILM
Correct exposure and adequate depth of field are the basic requirements of general underwater photography. It is worth considering the relationship between shutter speed, depth of field, and film speed. Under water, a good shutter speed is about 125th of a second. Things do not move very quickly under water, so there is no real need to increase the speed above this. Below this speed, one has to be careful to avoid camera shake. For more detailed aspects of this type of problem, readers are referred to standard texts on underwater photography (Mertens, 1970).
There are various speed ratings for various color and black and white films. In general, the faster the ISO (ASA) rating, the better the results, but at the expense of graininess. In normal situations of clear water and bright sunlight, medium-rated film of ISO 100–200 is normally used. With lower light levels, faster film will be required. Normally ISO 400 is the conven- tional limit for black and white film; beyond this, the film can be uprated or “pushed” to a higher ISO rating and then overdeveloped. Some special black and white films such as Ilford XP1 and Kodak T-Max have ISO ratings up to 3200 and can be pushed beyond this value, so that it is possible to take photographs in extremely low light levels. Kodak T-Max is particularly useful as it can be processed with conventional black and white processing. In most cases, color film is used for photographs that will be used for lecture and article illustration, and due to the increasing blueness of the color photograph with depth, artificial light is often needed to balance the color. For most recording purposes, black and white photographs are cheap and easy to produce in the field, and artificial light is only needed at very low light levels. With a little experience and by making some simple exper- iments, it is usually quite easy to determine the best film–developer com- bination. Provided that the light levels are monitored with an exposure meter, it is then quite straightforward to produce well-balanced and con- sistent results.
There are also some photographic films that are thinner than normal and thus allow more than the normal 36 exposures on a standard 35-mm film cassette. Ilford Autowind, with an ISO 400 film available in 72-exposure cassettes, has now been replaced by Ilford Type R550 Surveillance Film, available in 30-m rolls. Using this type of film can be advantageous when photographing a photomosaic, because it is possible to take twice as many photographs without having to reload the camera. Special developing





























































































   242   243   244   245   246