Film Reciprocity Failure Calculator
Correct exposure for film reciprocity failure during long exposures.
Supports common films (Tri-X, Provia, Velvia, HP5, Acros) for night photography.
Reciprocity Failure
Photographic film obeys the reciprocity law for “normal” shutter speeds: doubling exposure time and halving aperture (or vice versa) gives identical density. Below about 1 second, this breaks down. Silver halide grains stop accumulating image-forming photons efficiently, and the film acts slower than rated. The amount of time you actually need is longer than the meter says, and that extra time is the reciprocity correction.
Formula (Schwarzschild approximation)
t_corrected = t_metered ^ p
where p is the Schwarzschild exponent for the film, typically 1.10–1.50. A film with p = 1.0 has perfect reciprocity (digital sensors approach this). The closer p is to 1, the better the film handles long exposures.
Two things about that formula that catch people out. It takes the metered time in seconds, and it is not unit-agnostic: feed it 0.5 minutes instead of 30 seconds and you get a completely different answer. And it only applies at one second and above. Below a second it inverts, returning a shorter time than you metered, which is the opposite of what reciprocity failure does. Film obeys the reciprocity law well enough under a second that there is nothing to correct anyway, so the calculator leaves short exposures alone rather than pretending.
Common Film Exponents
| Film | Schwarzschild p | Notes |
|---|---|---|
| Fujifilm Acros 100 (II) | ~1.00 | Best-in-class, almost no failure to ~2 minutes |
| Kodak T-Max 400 | ~1.10 | Mild correction needed |
| Fujifilm Provia 100F | ~1.15 | Slight color shift on long exposures |
| Ilford Delta 100 / 400 | ~1.18 | Standard B&W choice |
| Kodak Tri-X 400 | ~1.30 | Heavy correction at >10 s |
| Ilford HP5+ | ~1.30 | Similar to Tri-X |
| Fujifilm Velvia 50 | ~1.40 | Major color shift; Fuji recommends strong correction |
| Lumiere historic film | ~1.50 | Severe failure |
Worked Example: Tri-X at Metered 30 s
- t_corrected = 30^1.30 = 30 × 30^0.30
- 30^0.30 = 2.774
- t_corrected = 83.2 s
You would actually expose for about 1 minute 23 seconds, which is 1.47 stops more, or nearly triple the metered time. Meter 30 seconds on Tri-X, shoot 30 seconds, and the frame comes back a stop and a half thin.
When to Apply Correction
| Metered Time | Correction Needed? |
|---|---|
| Under 1 s | None. The formula does not apply here and film does not need it |
| 1 to 4 s | Small but real on Tri-X and HP5, negligible on Acros |
| 4 to 30 s | Noticeable for most films |
| 30 s and up | Always correct, often substantially |
| 4 min and up | Essential, and cut development 10 to 20% as well |
Color Films
Color films have a separate reciprocity exponent for each emulsion layer. This produces color crossover at long exposures, typically a magenta or green cast, which can only partially be corrected with filters. For night architectural and astrophotography work, many photographers prefer Acros or modern T-grain films for this reason.
Limitations
The Schwarzschild model is an empirical fit, and real reciprocity curves can diverge above ~10 minutes. Always consult the film manufacturer’s data sheet for definitive corrections, and bracket your shots when in doubt.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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