Reciprocity Correction
Metered Time
4s
Corrected Time
6.1s
Additional Time
+2.1s
Stops Added
+0.6
Development Adjustment
-10% dev for 1-10s, -20% for >10s
Reducing development time helps prevent highlight blow-out from extended exposures.
Quick Reference for Ilford HP5+
| Metered | Corrected | Added | Stops |
|---|---|---|---|
| 1s | 1s | +0s | +0 |
| 2s | 2s | +0s | +0.3 |
| 4s | 6s | +2s | +0.6 |
| 8s | 15s | +7s | +0.9 |
| 15s | 35s | +20s | +1.2 |
| 30s | 1m 26s | +56s | +1.5 |
| 1m | 3m 33s | +2m 33s | +1.8 |
| 2m | 8m 49s | +6m 49s | +2.1 |
| 4m | 21m 52s | +17m 52s | +2.5 |
| 8m | 54m 14s | +46m 14s | +2.8 |
About Reciprocity Failure
- What it is: Film sensitivity decreases at long exposures due to the Schwarzschild effect
- When it occurs: Usually exposures >1 second (varies by film)
- Formula: Corrected time = Metered timep
- p value: Film-specific constant (typically 1.1-1.5; lower is better)
- Color shift: Long exposures on color film cause color crossover shifts
- Development: Reduce dev time 10-20% for very long B&W exposures to prevent blown highlights
- Best films for long exposure: Fuji Acros (p=1.1), Kodak T-Max (pā1.15)
Tips for Long Exposures
- B&W film: Generally handles reciprocity better; minimal color shift concerns
- Color negative: More forgiving than slide film; shifts can be corrected in scanning
- Slide film: Least forgiving; avoid exposures over 10s when possible
- Bracketing: Consider shooting multiple shorter exposures and stacking
- Testing: Run tests with your specific film/developer combination