The decreasing efficiency of photographic emulsions with longer exposure times. In daylight there is a reciprocity between exposure time and image brightness: a halving of the image brightness can be compensated for by doubling the exposure time. At low image brightnesses, however, this reciprocity fails and very much longer exposure times are needed. This is thought to be because the effects of individual photons on the grains of an emulsion are easily overcome by thermal motions. Cooling the emulsion in a cooled camera helps prevent this. Alternatively, the film may be hypersensitized.
Subjects: Astronomy and Astrophysics.