The reciprocity law in photography describes the inverse relationship between aperture and shutter speed when determining exposure. If you change one factor, the other must be adjusted proportionally to maintain the same exposure value.
For example, closing the aperture by one stop from f/8 to f/11 reduces the amount of light reaching the film or sensor, so you must double the shutter time from 1/60 sec to 1/30 sec to compensate. Conversely, opening the aperture or shortening the shutter speed by one stop also requires a counter adjustment. Using this principle, an exposure of f/8 at 1/60 sec could be equivalently set as f/11 at 1/30 sec, f/16 at 1/15 sec, f/5.6 at 1/125 sec, or f/4 at 1/250 sec. The law of reciprocity is fundamental for manual exposure control in both film and digital photography, allowing photographers to balance depth of field, motion blur, and light sensitivity creatively.