What Is a Reciprocity Failure Exposure Calculator?
In normal exposure, halving the light intensity and doubling the exposure time produce an identical result — this is called reciprocity. But at very long shutter speeds, most film stocks stop obeying this rule: their sensitivity drops, so the meter's suggested exposure time under-exposes the shot. This effect is called reciprocity failure, and this calculator corrects for it using each film stock's published reciprocity exponent.
The correction follows the Schwarzschild law, referenced on manufacturer data sheets for decades, where the true exposure time needed equals the metered time raised to the power of the film's reciprocity exponent (p).
How to Read Your Results
Adjusted Exposure Time
The actual shutter time you should use to correctly expose the shot, accounting for your film's reciprocity failure at long exposures.
Additional Stops Needed
How many extra stops of exposure the correction adds compared to your meter's original reading — useful context for how significant the failure is at your exposure time.
Exponent Used
The reciprocity failure exponent (p) applied in the calculation. Lower values (closer to 1.0) mean the film resists reciprocity failure well; higher values mean it needs much more correction at long exposures.
Real-World Example
A 10-second metered exposure on different black-and-white film stocks:
| Film stock | Exponent (p) | Adjusted time |
|---|---|---|
| Fujifilm Acros | 1.00 | 10.0 s (no correction) |
| Kodak T-Max 100 | 1.18 | ≈15.1 s |
| Kodak Tri-X 400 | 1.31 | ≈20.4 s |
The same 10-second meter reading needs roughly double the exposure time on Tri-X compared to Acros — a big enough difference to noticeably underexpose a long night shot if you ignore reciprocity failure.
Tips for Long Exposures on Film
- Reciprocity failure is usually negligible under about 1 second and becomes significant beyond several seconds.
- Always check the manufacturer's data sheet for your specific film stock's reciprocity characteristics — exponents can vary by batch and formulation.
- Long exposures may also need a color filtration adjustment on color film, since different color layers can fail at different rates.
- Digital sensors don't need this correction — reciprocity failure is a film-specific chemical effect.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team