Reciprocity
According to the reciprocity law, the density of a photographic image is directly proportional to the amount of light that enters the camera (the aperture setting), as well as the amount of time that the shutter is open (shutter speed). For instance, if the correct exposure for a subject is 1/125 of a second at f/4 and the aperture is increased by one stop to f/2.8, the shutter speed must be proportionally dropped by one stop to 1/60 of a second in order to maintain the same level of image quality, and vice versa.
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