Reciprocity Failure
Film's sensitivity genuinely weakens the longer light keeps hitting it, so a multi-second or multi-minute exposure needs more total time than simple doubling logic from a shorter test exposure would suggest — the reciprocal relationship between light intensity and exposure time that normally holds up perfectly breaks down specifically at these longer durations.
Digital sensors are essentially immune to this specific failure mode, though they hit their own separate long-exposure limits from thermal noise buildup instead. See the reciprocity failure answer page for the full correction formula and a genuine worked example.
Frequently Asked Questions
Does reciprocity failure affect every film stock the same amount?
No — the degree of sensitivity loss and the exact correction curve genuinely differ by film stock and manufacturer, which is why manufacturers publish stock-specific reciprocity data rather than photographers relying on one universal formula for every film.
Does reciprocity failure also shift color balance, or only exposure?
For color film, yes — different color layers in the emulsion typically lose sensitivity at different rates during a long exposure, producing a genuine, visible color shift alongside the exposure-time correction, which is why serious long-exposure film work sometimes needs a color-correction filter added alongside the extended exposure time itself.