It is easy to focus on a flat object, such as a slice of brain, but things get trickier 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. 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/16 routinely and reserve f/22 and f/32 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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