What is reciprocity and how it works?

[Martin Kapostas]

What is reciprocity and how it works?

Thnx

[Pete Andrews]

Reciprocity is the relationship between the lens aperture (image brightness) and the exposure time. For a given exposure of the film the time is the reciprocal of the brightness, and vice versa. If the aperture is opened up by one stop to double the brightness, then the time must be halved to maintain the same exposure. If the time is quadrupled, the brightness must be reduced to one quarter, etc. That's reciprocity, and how it works.

It's when it doesn't work that we are concerned. This is reciprocity law failure, to give it its correct name. It occurs with very short or very long exposures, and means that the expected exposure time must be increased to take account of a films non-linearity of response with respect to brightness. It varies from film to film, but it always results in underexposure if not compensated for.

[Ed Buffaloe]

If you are concerned about the failure of the reciprocity effect due to long exposures, you should consider using T-Max 100 film, as it has the least failure of reciprocity of any film on the market. At long exposures it is actually faster than T-Max 400.

[Garry Edwards]

Pete Andrews is 100% correct, but reciprocity law failure is not the problem that it was 10 years ago because films have improved so much.Unless the exposure time is ridiculously long, don't worry about it. If you are concerned, you can fairly safely rely on those bits of paper included with packs of tranny film for guidance on both exposure compensation and filtration compensation (which in my view is more significant, usually, than exposure compensation) or you can test on colour polaroid.