 BetterPhoto Member |
Reciprocity Failure What is reciprocity failure, and why does this occur when taking photoes at night without a flash?
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Jon Close |
In general there is a reciprocal relationship between light level and exposure time in getting the proper exposure recored on film. For example, a 1/2 reduction in the light level (either dimmer ambient light or smaller aperture) is completely offset by increasing the shutter time by x2, 1/8th as much light is offset by 8x longer shutter time. So for example 1/125 @ f/8 gives the same exposure as 1/60 @ f/11, 1/30 @ f/16, 1/250 @ f/5.6, 1/500 @ f/4, etc. But this relationship does not hold for longer exposures (usually 10 seconds and longer), or extremely short exposures (1/10,000 and less). For longer exposures to lower light levels f(or extremely short exposure to high light levels) the chemical sensitivity of the film loses this reciprocal relationship (suffers reciprocity failure) such that a 1/2 change in light level will require maybe a 3x, 4x, or greater increase in exposure time. Thus if the meter suggests 2 seconds at f/4 and the photographer wants to shoot at f/11 (3 stop change = 1/8th as much light), the reciprocal change to 16 seconds may not be enough and the film may require 30 seconds for proper exposure.
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