1.3 C. Exposure Time
Description
This article is from the Gross
Specimen Photography, by Ed Uthman, MD
1.3 C. Exposure Time
Exposure Time, or "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 : Always 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 ruin the picture by failing to expose part of the frame. Note: Some of the more modern and/or expensive cameras allow flash synching at 1/125 second or faster speeds, but make sure this is true of your camera before trying it. 1/30 sec - 1/2 sec : We tend to use this range for tripod or copy-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 copy stand, assuming you have a camera with an f/4 lens? A. ASA 50 film; f/4; 1/30 sec B. ASA 50 film; f/8; 1/8 sec C. ASA 200 film; f/16; 1/8 sec I would choose set-up 'B.' Set-up 'A' involves shooting at maximum lens aperture, at which lens resolution is not the best. Set-up 'C' lets us stop down the aperture for good lens resolution but requires us to use faster film with poorer resolution than the ASA 50. Therefore, 'B' looks like the best compromise. Even though a good copy 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 1/2 second, and you'd probably not notice it even if you shot a 2-second exposure (which may occasionally be necessary when using bellows at maximum extension; see below).
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