Expressed by (H)=Et, where E is the light intensity, and T is time. When E or T are varied to the extreme, an unsatisfactory exposure can result.

The theoretical relationship between the length of the exposure (shutter speed) and the intensity of the light (aperture) that dictates that an increase in one will be balanced by a equal amount of decrease by the other. This means that if you meter a scene and decide the exposure will be f/5.6 at 1/60 second you could obtain the same exposure by doubling your f-stop and cutting your exposure in half: f/4 at 1/125 second. Or you could do the opposite obtaining an exposure of f/8 at 1/30 second.

The reciprocity law states that Exposure = Intensity X Time. "Intensity" is the amount of light, and "Time" refers to how long that amount of light is allowed to act on the film's emulsion. Intensity is generally controlled by the aperture and time is generally controlled by the shutter speed. The reciprocity law therefore means that an exposure provided by Æ’8 and 1/250 sec will gave the same results as Æ’11 and 1/125 sec., or Æ’16 and 1/60 sec., and so on. If one choice of settings provides proper exposure, then the others will as well. The law "fails," however, when slow shutter speeds change the film's apparent speed characteristics, making it seem to have a slower speed and produce color shift.