The reciprocity law says that exposure depends on two things together: how much light reaches the film or sensor (set by the aperture) and for how long (set by the shutter speed). If you double one and halve the other, the exposure stays the same.
For example, if the correct exposure is 1/125 second at f/4 and you open the aperture by one stop to f/2.8, you must make the shutter speed one stop faster, 1/250 second, to keep the same exposure. With very long exposures on film this rule no longer holds, see reciprocity failure.
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