3.0 Image Size
Description
This article is from the Gross
Specimen Photography, by Ed Uthman, MD
3.0 Image Size
The size of the image in the camera depends on 1) the size of the subject (of course), 2) the distance of the subject from the camera, and 3) the focal length of the lens. The focal length is the distance from the lens to the image when the lens is focused on infinity. The effects of lens focal length are as follows:
The greater the focal length,
1. The larger the image appears for a given distance.
2. The farther away from the subject you can be for a given image size.
3. The more critical the damping of camera motion to prevent blurring.
4. The slower and more expensive the lens.
5. The less the sense of depth and perspective.
6. The less the curvilinear distortion of straight lines.
7. The _more_ flattering to the face in portrait photography (makes face less moony and nose less prominent).
8. The _less_ flattering to the body in figure photography (makes subject look stouter). Depth of field (see section II.B, above) is independent of focal length in the world of close-up photography [this is not true in landscape photography, where lenses with shorter focal lengths have greater depths of field].
Lenses are classified in groups based on their focal lengths and other properties:
16 - 35 MM (WIDE-ANGLE LENSES). Rarely used in medical photography, these are best for landscape and architectural photography. They make landscapes look more expansive and buildings more imposing. They tend to be extremely sharp lenses that have excellent contrast. 50 - 58 MM ("NORMAL" LENSES). These are used for most routine work, including gross photography. It is rarely necessary to use anything other than a normal lens for our purposes except when shooting close-ups so extreme that the bulk of the lens shadows the subject, so that it cannot be illuminated sufficiently. In this case you need: 80 - 135 MM (MEDIUM TELEPHOTO LENSES).These are used for high-magnification macrophotography to increase working distance, and for "over the shoulder" intraoperative photography. For instance, you can be twice as far away from the subject with a 100 mm focal length telephoto than with a 50 mm normal lens and still get the same image size on film. 200 - 2000 MM (LONG TELEPHOTO LENSES). These are usually not used in medical photography but are indispensable in sports, nature, and journalistic photography.
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