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330 Maritime Archaeology: A Technical Handbook, Second Edition
average light level of a normal scene is gray, this gives the correct exposure value.
An alternative metering system measures the incident light on a subject, i.e., the amount of light falling on the subject. Incident light can be mea- sured using either an incident light cone (or attachment) on a reflected light exposure meter, or a true incident light meter. Using this method, the meter is placed flat on the illuminated surface and is used to measure the light falling directly on the subject. This then gives the correct reading for any type of subject, and can be used to ensure that the lighting is even across the whole field of view.
When taking a photograph, there are three basic types of contrast situ- ations: dark, medium, and light subjects. It should be remembered that each requires careful consideration. In the case of a dark subject: Should the image of the dark subject be shown in its correct contrast or should it be artificially lightened by changing the contrast? For example, a ship’s timber is usually very dark with a lot of detail such as tooling marks, nail holes, etc., all appearing in low contrast (small variations in dark grays and blacks) on the almost black surface of the timber. If the light value is set according to the incident light, the details will be virtually lost because the timber will appear totally black against a correctly exposed, neutral gray background. If the reflected light value is gauged to determine the exposure, the mea- surement will assume that the black surface is, in fact, the 18% gray, and, as a result it will give a much longer exposure (thinking that the gray surface is poorly lit). The print will show the gray background incorrectly exposed as white, and the surface of the plank showing all the details as gray. By adjusting the development of the print (or in the case of digital images the levels), fine changes of contrast can be controlled, and thus all the required detail shown (Figure 12.3).
In the case of a neutral gray subject, the incident and reflected light readings will be the same. If the gray subject is surrounded by a highly contrasting background, the incident light reading will be correct, as will the reflected light reading provided it is taken off the surface of the subject.
The final case is that of the light subject such as a line drawing on white paper. Here a few black lines are depicted on an otherwise totally reflect- ing surface. This causes a high reflected light reading, and because it is getting high light levels, it assumes that the 18% reflecting gray surface is brilliantly lit. The resultant photograph is thus underexposed. The incident light meter is used in this situation because enhanced contrast (as was required in the case of the timber) is not needed. Alternatively, if the subject is a white object with a lot of light detail (say white lacework), the reflected light measurement is used.





























































































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