Reciprocity Exposure Calculator
Film loses sensitivity during long exposures, so a metered shutter time underexposes the frame: this is reciprocity failure. To compensate you extend the exposure using a power-law model where the corrected time is the metered time raised to a film-specific exponent. Because that exponent differs for every film stock and must come from the manufacturer's data, it is a user-editable input here. Enter your metered time and your film's reciprocity exponent to get the corrected shutter time.
Reciprocity formula
Corrected time = metered time ^ p
p = film-specific reciprocity exponent (> 1)
Extra stops = log2(corrected / metered)
The exponent p is supplied by the film manufacturer's reciprocity data and entered above. With p = 1 there is no failure and the corrected time equals the metered time. Times must be in seconds for the power law to behave consistently.
Worked example
A metered time of 10 seconds with a film exponent of 1.3: corrected = 10 ^ 1.3 = 19.95 seconds. Extra time = 19.95 - 10 = 9.95 seconds. Extra exposure = log2(19.95 / 10) = 1.00 stop. You would set roughly a 20 second exposure instead of 10.
Reciprocity exposure: frequently asked questions
What is reciprocity failure?
Film responds less efficiently to very dim light over long exposures, so the simple rule that exposure equals intensity times time breaks down. To get the intended density you must extend the metered time. Digital sensors do not suffer this; the effect applies to film.
How is the corrected time calculated?
A common model raises the metered time to a power: corrected = metered ^ p, where p is a film-specific exponent greater than 1. This calculator leaves the exponent as a user-editable input because each film stock differs; consult your film's published reciprocity data and enter its value.
Why is the reciprocity exponent an input rather than fixed?
Because reciprocity behaviour varies by film emulsion, and the exact correction is specified by each manufacturer's datasheet, no single number is correct for all films. This tool ships the exponent as a clearly labelled input so you supply the value from your film's documentation rather than relying on a guessed constant.
Sources
- NIST: Time and Frequency Division (the second).
- The power-law correction is a standard model; the exponent is film specific and is supplied by you from the film datasheet, never assumed.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.