Film Reciprocity Failure Calculator
Correct a long film exposure for reciprocity failure using the Schwarzschild exponent for popular black-and-white films.
How it works
Corrected time = metered time ^ p, where p is the film’s Schwarzschild exponent · Extra stops = log₂(corrected ÷ metered)
At very short and very long exposures film stops responding linearly to light — the "reciprocity" between time and intensity fails, so long exposures come out underexposed unless you add time. The Schwarzschild model raises the metered time to a power p, which is close to 1 for reciprocity-corrected films and larger for films that fail badly. Modern T-grain films like T-Max are excellent (p near 1.06), while older emulsions need much larger corrections.
Worked example
A meter reads 10 s on Ilford HP5 Plus (p = 1.31). The corrected time is 10^1.31 = 20.4 s, so you add about 10 seconds — roughly +1 stop of compensation.
Frequently asked questions
When does reciprocity failure matter?
Mainly for exposures longer than about one second — night scenes, dim interiors and pinhole photography. Below a second the correction is negligible for most films.
Where do the exponents come from?
They approximate each manufacturer’s published reciprocity data with a single power law. They are close but not exact; when precision matters, use the film’s own correction table.
Does this apply to digital?
No. Digital sensors do not suffer reciprocity failure the way film does — long exposures only add thermal noise, not underexposure, so no time correction is needed.
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This calculator is for educational purposes. Double-check important results before acting on them.