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.
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 ≈ 2.71
- t_corrected ≈ 81 s
You’d actually expose for about 1 minute 21 seconds — nearly triple the metered time.
When to Apply Correction
| Metered Time | Correction Needed? |
|---|---|
| 1/8 s and faster | None |
| 1/2 to 1 s | Mild — 10% bump for slow films |
| 4 s | Noticeable for most films |
| 30 s+ | Always correct, often substantially |
| 4 min+ | Essential — and reshoot if uncertain |
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 — that 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 — 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.