How reciprocity correction works
Film becomes less efficient during long exposures. Enter the time from your light meter, then use the corrected time as the total shutter time. This calculator uses seconds internally, including for exposures longer than a minute.
Ilford and CineStill models use corrected = meteredp. Published point tables from Kodak, Fujifilm, and Foma use the same log-log interpolation as FilmRecip. Results past the last published point are labeled as extrapolated.
Sources
Coefficients, tables, thresholds, and the 1.3 fallback were copied from FilmRecip's data definitions on September 2, 2026.
Questions
What is the HP5 reciprocity formula?
Ilford's published formula is corrected time = metered seconds raised to 1.31. A metered 10-second exposure becomes 20.4 seconds.
Does Portra 400 have an official long-exposure table?
No. Kodak says no correction is needed through one second and recommends testing for longer exposures. FilmRecip labels its longer result as a community estimate.