2.0 Focus
Description
This article is from the Gross
Specimen Photography, by Ed Uthman, MD
2.0 Focus
There are two things to consider here, methods of focusing and depth of focus.
A. Methods of focusing. 1. Autofocus. Most manufacturers today produce autofocus cameras aimed at various markets. The most popular of these, aimed at the advanced amateur and the professional, are probably the Minolta Maxxum series and the Canon EOS. These cameras are packed with automation which allow automatic film advance and rewind, automatic and program exposure modes, and autofocus. Automatic focusing uses a system whereby a computer in the camera uses vertical lines in the subject and focuses the lens by analyzing these lines. I have not used autofocus systems in specimen photography but have experience with them for snap shooting. The problem is that if there are insufficient vertical lines in the picture, the focusing system with be fooled and can leave you with a terribly out-of- focus picture. I have stuck with manual focusing for specimen photography but would love to hear what the autofocus aficionados have to say about its use. 2. Manual focus. In this method you simply view the subject through the viewfinder and turn a focusing ring until the subject sharpens. If you have a choice, I recommend a viewfinder with a split-field focusing prism to help with critical focusing, but others prefer a focusing grid, which, as far as I know, is only available on high-end cameras, like the Nikon F series. B. Depth of field It is easy to focus on a flat object, such as a slice of brain, but things get stickier when photographing objects with depth, such as a windowed pediatric heart specimen. Shooting these subjects requires a knowledge of the concept of depth of field. It turns out that the zone of depth at which the camera is in focus is greater at smaller apertures (larger f/ numbers) than at larger apertures. Therefore focusing is very critical when the lens is "wide open" but much less so when "stopped down." Let's say you are shooting an opened colon to demonstrate, en face, a large villous adenoma. If you focused on the "top" of the tumor (the part nearest the camera) and shot the picture with the lens aperture at f/2, the tip of the adenoma would be in focus, but the sides would be slightly out of focus, and the surrounding colonic mucosa would be totally out of focus and probably not recognizable. However, if you stop down to f/16, the entire specimen would be in focus. Since this results in decreasing the exposure by six stops, you would have to compensate by increasing the exposure time by a factor of two to the sixth power, or 64. For good depth of field and optimal lens resolution, I use f/8 routinely and reserve f/16 and f/22 for subjects like the windowed heart. Most cameras have a "depth-of-field preview button" that lets you stop down the lens to its preset aperture, so you can view how much depth-of-field you'll end up with in the resulting picture (normally the aperture diaphragm stays wide open until the instant the picture is taken, so you have a nice, bright viewfinder in which to compose the shot).
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