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Understanding reciprocity

Techniques

24 October 2007 14:12

When light levels drop, the normal reciprocal relationship between shutter speed and aperture breaks down.

In regular daylight conditions, the relationship between the amount of light and the exposure time is constant. So if the light levels halve, you need to double the exposure time to get the same result. However, this rule – known as the reciprocity law – fails when using film in very low light levels.

Once the exposure time becomes longer than a few seconds, the film no longer reacts in the same way – responding with colour shifts and progressive underexposure. This characteristic is known as reciprocity law failure. The precise amount of failure varies between different films, but as a general rule of thumb you need an extra half-stop for exposures from 1 to 10 seconds.

Up to 30 seconds, add 1 stop, and beyond that, another 2 stops for longer exposures. So, for example, if your camera indicates it needs an exposure of 15 seconds, the exposure needed will actually be 30 seconds to ward off reciprocity law failure.