Page "Reciprocity (photography)" Paragraph 8

For most photographic materials, reciprocity is valid with good accuracy over a range of values of exposure duration, but becomes increasingly inaccurate as we depart from this range: this is reciprocity failure ( reciprocity law failure, or the Schwarzschild effect ).

As the light level decreases out of the reciprocity range, the increase in duration, and hence of total exposure, required to produce an equivalent response becomes higher than the formula states ; for instance, at half of the light required for a normal exposure, the duration must be more than doubled for the same result.

Multipliers used to correct for this effect are called reciprocity factors ( see model below ).