The Reciprocity Effect

Ordinarily, f/stops and shutter speeds have an equal effect on exposure. Changing the f/stop from f/8 to f/11 is equivalent to changing the shutter speed from 1/60 to 1/125 of a second. In either case, the exposure is one stop less. This is because f/stops and shutter speeds are calibrated to be equivalent, or "reciprocal." Unfortunately, this is true only within a certain range of exposure times (camera shutter speeds). If your exposure is longer than 1/2 of a second or shorter than 1/1000 of a second, the reciprocity rule "fails."

For example, a negative exposed for 1/2 of a second at f/2.8 will give lower densities than one exposed for four seconds at f/8. The film will be underexposed even though, according to your light meter, these two exposures should be the same. This is called reciprocity failure. Assuming that you need to use f/8 for depth of field, the solution would be to use an exposure of f/8 at 1520 seconds. If this sounds imprecise, you will find that published reciprocity charts are very general. It's also necessary to compensate for the increase in contrast that results from such long

exposures. Ten to thirty percent less development will be required for negatives that have been exposed for longer than 1/2 of a second.

Because of their unique grain structure, Kodak TMax films require less compensation for the reciprocity effect than conventional films. As stated above, with standard grain films, when your indicated exposure is 10 seconds, a 50-second exposure is required, with a 20 percent reduction in your development time to avoid underexposure and overdevelopment. With TMax, Kodak recommends an exposure of only 15 seconds for a 10-second indicated exposure with no reduction of your development time. Standard reciprocity failure charts apply to Ilford's Delta films.

For more information on the reciprocity effect, consult Kodak Professional Black and White Films data book, or Photographic Materials and Processes published by Focal Press.

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