Glossary
Reciprocity failure
Also called: reciprocity, reciprocity law failure
Film loses sensitivity at very long exposures, so the meter reading is no longer enough.
The reciprocity law says doubling time and halving light should give the same exposure, but for exposures longer than about one second, film stops obeying. A nominal 30-second exposure on Portra 160 might actually need 60 seconds. Each stock has its own reciprocity curve. The app applies the right correction automatically once you select a film stock and the exposure is in the affected range.
Where this shows up in the app
How the Reciprocity Failure Calculator works
Adds the extra exposure film needs at long shutter speeds.
Applied automatically inside the meter when the loaded film stock has a non-trivial reciprocity exponent and the shutter is past the threshold.
Reciprocity Crossover Alert tied to ephemeris
Predicts the shutter at every twilight event and warms the chip when reciprocity bites.
Tools tab > Sun & Moon, second line under each event row