Long exposures and the one-line reciprocity formula

At very low light levels film gets less sensitive: double the time and you get less than double the density. That's low-intensity reciprocity failure, and it matters for night scenes, pinhole cameras and anything on a tripod past a second.

Ilford's method

Ilford replaced its graphs with a single factor per film and one formula:

corrected time = metered timeP

Times are in seconds. Anything of one second or less needs no correction. Their own example: HP5 Plus metered at 10 seconds gives 10^1.31 = 20.4, so expose for 20 seconds.

Film	P	10 s metered	60 s metered
SFX 200	1.43	27 s	5:48
Pan F Plus	1.33	21 s	3:51
Delta 100	1.26	18 s	2:53
Delta 400	1.41	26 s	5:21
Delta 3200	1.33	21 s	3:51
FP4 Plus	1.26	18 s	2:53
HP5 Plus	1.31	20 s	3:33
XP2 Super	1.31	20 s	3:33
Ortho Plus	1.25	18 s	2:46
Kentmere 100	1.26	18 s	2:53
Kentmere 400	1.30	20 s	3:24

The calculator works it out for any time.

Two warnings from the same sheet

• For very long exposures, the accuracy of your light reading starts to matter as much as the formula. Expect some trial and error, and bracket.
• Contrast tends to rise with long exposures, because highlights and shadows suffer different amounts of reciprocity failure. You may need to reduce development time.

Kodak films

Kodak gives its own adjustments in each film's datasheet (Tri-X is F-4017, T-Max 100 is F-4016, T-Max 400 is F-4043). I haven't included them here.

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