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 ƒ8 and 1/250 sec will gave the same results as ƒ11 and 1/125 sec., or ƒ16 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.
What about digital photography where no film is used. So far, I have not encounter RF yet though I do a lot of night photography.