Reciprocity Failure
The breakdown of the reciprocity law at very long or very short exposures, where film needs more light than calculation predicts.
Formula / rule
t_corrected = t^p
Measured in
seconds
In depth
At low light levels, the few silver atoms formed by scattered photons can revert before a stable latent image cluster builds, so the film effectively loses speed. Each emulsion has its own correction curve, and beyond about a second the required increase grows rapidly — a metered ten seconds may need a minute or more. Colour films also shift colour as the layers fail at different rates.
Examples in practice
A metered exposure of 30 s on one popular film actually requires around four minutes once reciprocity is corrected.