Correct long film exposures for reciprocity failure and the added exposure time.
Beyond a few seconds, film reacts non-linearly, so the corrected time is the metered time raised to the reciprocity factor. Without correction, long exposures come out underexposed, frustratingly so for astro and night work on film.
Reciprocity Failure Correction
Corrected = exposure^reciprocity factor
Corrected = exposure^reciprocity factor Beyond a few seconds, film reacts non-linearly, so the corrected time is the metered time raised to the reciprocity factor.
Without correction, long exposures come out underexposed, frustratingly so for astro and night work on film.
This calculator takes 2 inputs: Metered exposure, Reciprocity factor. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.