Film loses sensitivity in long exposures, so a metered 10-second exposure may need 20 seconds or more. Ilford publishes a simple power formula for its black-and-white films; enter your metered time and film to get the corrected exposure.
Applies to exposures longer than one second.
How it works
T_corrected = T_metered^p (Ilford technical data, T > 1 s).
FAQ
Does digital have reciprocity failure?
No — digital sensors respond linearly, though very long exposures add thermal noise.
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