Reciprocity
Ilford Delta 400 Professional long exposures
What a metered time becomes on Delta 400 Professional, 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 = 1.41)
At a glance
| Metered | Expose for | Added |
|---|---|---|
| 1 s | 1 s | — |
| 2 s | 2.7 s | +0.4 stops |
| 4 s | 7.1 s | +0.8 stops |
| 8 s | 19 s | +1.2 stops |
| 15 s | 46 s | +1.6 stops |
| 30 s | 2 min 01 s | +2 stops |
| 1 min | 5 min 22 s | +2.4 stops |
| 2 min | 14 min 14 s | +2.8 stops |
| 4 min | 37 min 50 s | +3.2 stops |
| 8 min | 1 h 41 min | +3.7 stops |
| 16 min | 4 h 27 min | +4.1 stops |