Page 350 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 12: Artifact Photography 329
for 24 or 70 exposure backs), of a considerable increase in cost of the unit, and of bulk. One advantage with some 57-mm square format cameras is that they have interchangeable film magazine backs. This is very practical as different film types can be held in different backs and with this format can be changed rapidly, without having to move the camera, thus saving expensive camera duplication. This option is unfortunately only available on one 35-mm camera system (Rollei SL).
In archaeological work, the main requirement for a large format is to copy large-scale plans. This is best done with either a monorail camera, or on a reproback on an enlarger system, using 120¥ 100-mm format sheet film (13 times larger than the 35-mm system). In the reproback enlarger system, very accurate alignment of the camera axis with the plane of the subject ensures that there is no tilt in the system. Such systems are not portable and tend to be used only at the homebase for copying line work.
Digital cameras are now a serious contender in the range of ways of taking pictures and, as has been described in Chapter 10, the use of scan- ners overlaps this area too. The digital camera is now capable of producing images that are as good or almost as good as the conventional camera. There are two basic types: the simple system with a zoom lens and rela- tively self-contained system; and the complex camera, essentially an SLR camera with a digital back and usually very expensive. The enormous advantage with the digital system is that it obviates the need to process film and as such is part of the evolving digital photographic revolution.
B. EXPOSURE METERS
The exposure meter is an instrument used to measure either the light incident on, or the light reflected from the subject. The meter can be either hand-held or, as previously mentioned, located inside the camera. In the latter case, it can only measure the reflected light. Some through-the-lens metering systems give a weighting to the lower center of the photograph, which is usually the area required to be correctly exposed. The bias to the lower part is to reduce the effect of the upper part of the photograph which is generally the sky. It is necessary to consult the instruction manual for the particular camera, as these systems vary.
Reflected light metering assumes that the object has an 18% reflecting surface. It measures the light reflected from the surface of the object and calculates the exposure accordingly. In normal scenes, and in open-air photography, the range of subject reflection is considerable, so that the integrative effect works well giving an average value between the extremes. In other words, the meter assumes that the subject is gray and, as the