1.4 How do you determine exposure?
Description
This article is from the Gross
Specimen Photography, by Ed Uthman, MD
1.4 How do you determine exposure?
There are two ways to do this 1. Most cameras have a built-in light meter that monitors the amount of light coming through the lens. This meter attempts to optimize the exposure either by averaging the total light hitting the film plane (an "averaging meter") or using a small sample area (usually the center of the field) to measure the amount of light focused on that particular spot (a "spot meter"). In an "aperture priority" system, the meter then looks at the aperture you have set on the lens and automatically adjusts the shutter speed to give the desired exposure. In an "shutter priority" system, you set the shutter speed and the light meter automatically adjusts the aperture. These functions are available in what is referred to generally as the "auto" mode. In addition, most modern cameras have a "program" mode, which completely automates exposure determination by choosing both the aperture and the shutter speed for you. This means all you have to do is compose the picture, focus, and push the button. Program mode has been a boon for photography in general, because it allows you to concentrate on composition and not have to worry about fiddling with aperture rings and shutter speed knobs. There is, however, a price to pay, especially in technical photography. The main problem is that automatic exposure systems (except in high-end cameras) are standardized for snapshot type photography, where there is no striking difference between background and subject illumination. Also, an automatic exposure system will attempt to make the subject have a "neutral" brightness. In technical photography, we do not necessarily want this; we want brain to look light and spleen to look dark, just like these respective subjects appear to us in real-time. Therefore, I do not use the camera's automatic exposure system for routine specimen photography. 2. Because of the above considerations, I recommend that you take advantage of the rigidly standardized exposure environment of the copy stand and virtually always use manual exposures. Determine the ideal exposure by shooting a roll of film at various settings and then stick with this exposure when shooting specimens. You can still use the light meter when faced with an unstandardized situation, such as having one of your four floodlights burn out on Saturday and not being able to find a replacement. Parenthetically, I have found through experience that when shooting documents of black printing on white paper, you should use an exposure one stop brighter than your standard setting for specimens. For instance, if you normally shoot specimens at f/8 and 1/8 sec, you should choose f/8 and 1/4 sec when shooting a document. Never, never let the camera shoot black-on-white printed documents on "Auto" or "Program," because the camera will think you want the white paper to appear neutral and will force a bad underexposure. Another hint: When forced with shooting pictures on a set-up you are unfamiliar with, you may have no idea what settings to use. A good solution is to meter on the palm of your hand (believe it or not, it makes no difference what color you are; the palm of everyone's hand looks about the same to a light meter) and note what settings the camera's light meter indicates. Simply switch over to manual and enter these settings. Then you can shoot away and always get at least acceptable results.
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