Reciprocity
Ilford FP4 Plus long exposures
What a metered time becomes on FP4 Plus, and the stops it adds — worked out from Ilford’s own data, never past it.
Your metered time
Tc = Tm^P for metered times over 1 s; 1 s or less needs no correction (P = 1.26)
At a glance
| Metered | Expose for | Added |
|---|---|---|
| 1 s | 1 s | — |
| 2 s | 2.4 s | +0.3 stops |
| 4 s | 5.7 s | +0.5 stops |
| 8 s | 14 s | +0.8 stops |
| 15 s | 30 s | +1 stops |
| 30 s | 1 min 13 s | +1.3 stops |
| 1 min | 2 min 54 s | +1.5 stops |
| 2 min | 6 min 57 s | +1.8 stops |
| 4 min | 16 min 38 s | +2.1 stops |
| 8 min | 39 min 50 s | +2.3 stops |
| 16 min | 1 h 35 min | +2.6 stops |