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P.A. Program - Gross Specimen Photography
EXPOSURE TIME
"Shutter speed," measured in seconds or fractions of seconds (1/30 s, 1/1000 s, etc) represents the total time the film is exposed to the focused image. It is determined by setting the camera shutter to open for a specified length of time.
Effects of various shutter speeds:
1/1000 sec - 1/60 sec: These are OK for hand held camera in existing light.
1/60 sec : Use this with electronic flash, since just about all flashes are specifically synchronized for this speed. Using a slower speed (e.g., 1/30 sec) will also work, but a faster speed (e.g., 1/125 sec) will fail to expose part of the frame.
Some cameras allow flash synching at 1/125 second or faster speeds, but make sure to check your camera before trying it.
1/30 sec - 1 sec : We tend to use this range for tripod or camera-stand work, including gross photography. This range is generally not acceptable for hand-held cameras, because most people cannot hold the camera still enough for this length of time. By using these slower speeds for gross photography, we allow ourselves the luxury of smaller apertures (giving us good depth of field and maximum resolution from the lens) and slower films (giving us maximum film resolution and best color rendition).
For example, each of the following exposure parameter set-ups give the same exposure. Which would you choose for a gross photograph taken on your camera stand, assuming you have a camera with an f/4 lens?
A. ISO 160 film; f/4; 1/30 sec
B. ISO 160 film; f/8; 1/8 sec
C. ISO 160 film; f/16; 1/2 sec
I would choose set-up ‘C.’ Set-up ‘C’ lets us stop down the aperture for good lens resolution.
Even though a good camera stand will keep the camera motionless and allow long exposure times, there is a theoretical problem, called "reciprocity failure," which may interfere with color balance in very long exposures. But this is never a problem as long as you don’t allow the exposure time to exceed 1 to 2 seconds.
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, many 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 camera 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 lights burn out on Saturday and not being able to find a replacement.