1.2 B. Aperture
Description
This article is from the Gross
Specimen Photography, by Ed Uthman, MD
1.2 B. Aperture
Aperture, the setting of the iris diaphragm in the lens, determining how much light is allowed through the lens into the camera. Aperture measured as "f/ stops" (f/2.8, f/4, f/16, etc). The f/ ratio is calculated by dividing the focal length of the lens (see below) by the diameter of the iris diaphragm opening through which light passes. Therefore, the greater the diameter, the more light is let in, and the smaller is the f/ ratio. Each f/ stop is 1.4 (the square root of 2) times the preceding f/ stop. Each "stop" multiplies the amount of light by 2X. As an example, f/1 lets in twice as much light as f/1.4 and four times as much as f/2. The "speed" of the lens is its f/ ratio at its widest aperture setting. An f/1.2 lens is considered very "fast," while an f/5.6 lens is "slow." Generally, fast lenses are more expensive than slow ones and in fact do not have as good corner-to-corner resolution as slower lenses. Because we generally have plenty of light at our disposal in gross photography, we opt for excellent resolution over lens speed. Most lenses for our purposes are f/2.8 to f/4 at their widest aperture settings. We typically choose to "stop down" our diaphragms in most cases, because almost all lenses have optimal resolution when not used at their maximum aperture. The "ideal" f/ stop is generally taken as 2 to 2-1/2 stops "down" from the maximum aperture. For an f/2.8 lens, therefore, the optimal aperture setting would between f/5.6 and f/6.7. The other reason to stop down from maximum aperture is to improve "depth of field" (see "Focus," below). I personally shoot almost all my specimen photos at f/8.
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