Reciprocity Explained

-Excerpts taken from Kodak TECHNICAL PUBLICATION DATA I FILM E31- July 2002

The reciprocity law states that the intensity of light falling on a photographic film multiplied by the exposure time equals the total amount of exposure.

Intensity x Time = Exposure

This means, for example, that an exposure of f 16 at 1/60 second is equivalent to an exposure of f 11 at 1/125 second. In either combination of settings, the same total amount of light reaches the film. The reciprocity law applies to commonly available black-and-white and color films at exposure times from approximately 1/10 second to 1/1,000 second. This means that no adjustments are necessary for exposure or color in this shutter range. Most modern films however have increased this range to the point that no adjustments are necessary from approximately 1 second to 1/10,000 second. It should be pointed out that all photographic emulsions are subject to an effect often called “reciprocity-law failure.” At exposure times at the edges of the above ranges, you will begin to see underexposure (loss of effective film speed) at the normally calculated exposure setting, a change in contrast, a color shift, or a combination of these effects. The word “failure,” in this context, does not imply a short coming of the film, flash equipment or the camera, but merely that the reciprocity law does not hold for very long or very short exposures times.

We also sometimes refer to these changes in film response to particular illumination levels as “long-exposure effects” and “short-exposure effects.”

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