Overview of reciprocity law

Opening a larger aperture allows more light to reach the film through the lens than shrinking a smaller aperture, and a lower shutter speed allows light to stay on the film for a longer time than a higher shutter speed. From this, it can be evolved into the following situation: the combination of aperture f/2.8 and shutter speed 1/500 seconds, and the combination of aperture f/4 and shutter speed 1/250 seconds have the same exposure effect. They are also consistent with the exposure effect obtained by the middle aperture f/8 combined with the medium shutter speed of 1/60 seconds. This is called equal exposure.

All films with equal exposure have the same negative density after development. In the exposure formula, the illuminance and time values can be replaced with each other. For example, the illumination is 100 lux, and the daily exposure is 1 1 00 seconds, which is equivalent to the illumination is1lux and the exposure is1second. This relationship that illumination and time can be substituted for each other is called reciprocal law in photographic exposure, also known as reciprocal law or reciprocal law.