Film Reciprocity Failure Calculator
Pick your film stock and enter the time your light meter gives you. At long shutter speeds film under-exposes because the light-sensitive reaction slows down — this tool tells you the extra time you actually need to set on your camera. Nothing uploaded.
Exposure Bracket
| −⅓ stop | Corrected | +⅓ stop |
|---|---|---|
| — | — | — |
Learn more: film reciprocity failure and exposure correction
Why long exposures need extra time
Ilford's technical sheet on reciprocity failure compensation describes it as the effect where giving the same total exposure over a longer time produces a lower image density. It says exposures of one second or less need no compensation.
The calculator uses the same power-law form Ilford publishes: corrected time = metered time to the power of 1/p. Ilford writes it as Tc = Tm^P, so P in its table is 1/p here. Ilford's own example is HP5+ at 10 seconds metered, which gives 10^1.31 = 20.4 seconds.
The difference between films
Take a 30 second metered exposure. HP5 Plus (Ilford factor 1.31) needs 86 seconds, about 1.5 stops more. FP4 Plus and Delta 100 (factor 1.26) need 72.5 seconds, and Delta 400 (factor 1.41) needs 121 seconds.
The Ilford values in the calculator come from that sheet. The values for other brands are approximations, so treat them as a starting point and bracket your exposures. The calculator also has a cold-temperature option that adds 10% to the corrected time.
Brackets, limits and color
The bracket suggestion gives the corrected time, one third of a stop shorter and one third of a stop longer. That is a factor of 2^(1/3), about 1.26, on either side. Each film also has a maximum metered time, shown as "beyond reliable range" when you pass it.
Ilford also notes that contrast can rise on long exposures, and that pulling development may help. Time correction does not fix color shift on color film, so the calculator's film notes warn about it on Velvia.
FAQ
Why is there no correction at 1 second or less?
Ilford says exposures of one second or less need no compensation, and the calculator returns the metered time unchanged for anything at or under 1 second.
What does the bracket suggestion give me?
It gives three exposure times: the corrected time and one third of a stop either side of it. Correction is an approximation and film varies slightly by batch, so three frames are safer than one.
Is the formula exact for every exposure length?
No. Ilford notes that very long exposures at very low light levels bring other variables, so some trial and error may be needed. The calculator marks results past each film's maximum time as beyond the reliable range.