Reciprocity How it may affect monochrome film

The use of a Zero pinhole camera means that long exposures are the norm. It might be a good idea to give an insight into how reciprocity affects the negatives. It is a wide-ranging subject that afflicts colour and black and white materials in different ways at long exposures, extremely short exposures and with flash. Because of this I am limiting this post to long exposure times with black and white film.

The Law:

The formula E=IxT expresses reciprocal relationship between the intensity of light reaching the film and the time allowed to act on the film. If one increases the other decreases proportionally, no net change in exposure occurs.

Ansel Adams The Negative.

At long exposures this law breaks down, known as reciprocity failure. It is where a seconds worth of light is not enough to satisfactorily produce the required densities in the negative. So the longer the exposure the greater the compensation needs to be. With these exaggerated times comes a side effect in that the lower values recorded become under exposed more than the higher ones causing the contrast of the negative to increase. You can make adjustment for this when developing the film by a reduction of ten percent for up to ten seconds, from ten seconds by twenty percent and from a hundred seconds by thirty percent. These changes are easy to control for single frame exposure made with large format cameras but a little difficult to achieve with roll film. If all the frames on the film are exposed for no longer than ten seconds then the ten percent reduction will control the increase in contrast across the whole film, but in practice this is not always the case with exposure times being all over the place. I would suggest that a reduction of ten percent be the starting point and that with experimentation will find what works best for you. Having said all this the side effect of higher contrast maybe to your liking in which case where's the problem.

Further reading

The picture included in this post shows a slight increase in contrast. The negatives Ilford FP4+ were processed in PMK Pyro without an adjustment for increased contrast. Printed on grade two multi grade paper developed using Moersch 6 blue.

The following images and those above were all made using a Zero multi format Pinhole camera. The pictures below are from negatives using Fomapan 100 at box speed developed in studional for 11 minutes. scanned from photographs printed on Ilford multigrade 4 developed in souped ilford multigrade.