PHOTO 2 Pgy 4410

Correcting for film reciprocity failure

In ideal world, if the light intensity doubles, one can always halve the exposure time to obtain identical exposure on photographic materials. Or, when the light intensity halves, one can double the exposure time to compensate. This is the "reciprocity law" which usually but not always applies to real world materials; there are cases where real materials fail to obey this law. The failure is seen when light intensity is extremely low as in nightscape photography, or extremely high as in some flash exposures. Although the degree of this phenomenon is governed by the light intensity of the exposure, it is often described in terms of exposure duration for practical photography purposes. This simplification works as long as you are measuring incident light (or reflected metering off standard gray card) and aiming at normal exposure level. In this article, we use intensity for theoretical discussions and exposure duration for practical aspects.

Kodak Reciprocity Correction Data for Black and White Films

Reciprocity law failure

Night Photography Part 1, 2 & 3