Reciprocity failure.
Film appears to become less sensitive to light when given exposures of a second or so or longer. This is reciprocity failure or, more accurately, failure of the law of reciprocity.
The rule of reciprocity only works within a certain exposure time range for any given film (or paper, though this is usually only matters when using film). At very high shutter speeds and very low shutter speeds reciprocity breaks down and exposure time is no longer reciprocal to exposure intensity.
This is most often seen with long time exposures - low-intensity reciprocity failure (LIRF) induced by the presence of water and oyxgen in the emulsion. When doing, say, night photography where exposures of several minutes are common, you’ll find that you need to expose film for much longer than you’d expect. This is because at such slow speeds reciprocity has failed and you need to compensate by providing additional exposure time - often up to 50% more.
It’s a particular problem in colour photography since colour film relies upon 3 or 4 independent layers of emulsion, each sensitive to a different colour. These layers react to light in slightly different ways and the point at which reciprocity fails for each layer is going to be slightly different. So really long shutter speeds can result in strange colour shifts when one layer responds very differently from another. Each brand and type of film has different response curves and so the only way to figure out the characteristics of a given film is to experiment.
Note that I said that reciprocity failure occurs at high shutter speeds as well. In the past this was a problem with flash photography - the burst of light from an electronic flash was often too fast for the film emulsion to react adequately. This isn’t really a problem anymore, however. Modern flash units tend to have longer flash bursts and modern film emulsions also react to light better at faster speeds.
Note also that reciprocity failure isn’t an immutable problem. It’s possible to put film into a vacuum in order to drive water and oxygen out of the film and then apply hydrogen gas - a technique known as hypersensitization or hypering.
cf. exposure value (EV), hypersensitisation, reciprocity, response curve, shutter speed.
Entry last updated 2002-05-06. Term 1004 of 1487.
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