Page 247 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

226 Maritime Archaeology: A Technical Handbook, Second Edition
uations because they often cause halation in the black and white negative. It is worth briefly considering this problem because it gives an insight into some of the technical difficulties of underwater photography.
Halation is the result of local overexposure causing the light to irradiate from the light areas into the darker surroundings of the photographic emul- sion, giving the effect, on the print, of a halo around a white object. It is usually caused by the film backing and is most obvious in black and white film. It occurs to some extent with color film, but not at all with digital pho- tography. In many cases, halation is simply aesthetically unpleasant; however, with scales it causes the graduations between black and white to be unequal in length. Usually, under water at low light levels there is no natural pure white, only a range of mid-grays to black can be observed. In order to get the correct exposure, a reflected light exposure meter is used. This results in the photograph being overexposed, but produces the full tonal range in gray. If the black and white scale was not in the picture, the results would be perfectly acceptable. If, however, small areas of pure white exist, which do not contribute to the integrative effect of the exposure meter, the white areas will be grossly overexposed relative to the gray, thus creating the halation. This effect can be overcome by selecting a suitable tone for the light part of the scale. Light gray is best, although a colored paint can also be used. The objective is to tone down the white, thus reduc- ing the contrast between the light and dark graduations. It should also be noted that with digital cameras halation is not a problem.
IV. GENERAL FIELD PHOTOGRAPHY
General field photography covers all aspects of the non-underwater expedition work. It includes coverage of the preparation on the surface for the underwater work; it also involves recording the field conservation and any work related to the artifacts of a general nontechnical nature. The tech- nical problems related to artifact photography are dealt with separately in Chapter 12. Essentially, general field photography covers the type of pho- tographs that may be required to illustrate a talk or report on the field work or that can be used for reference purposes at a later date.
A great deal of care has to be exercised in these situations to ensure that the photographic recording does not get out of control. Large numbers of photographs can be taken of the running of the expedition, but they often lack purpose or meaning. An alternative is to request that the expedition be allowed to obtain copies of any personal photographs taken by team members during the field operations. This way, the best coverage can be selected from a wide range of material.