R Reciprocity 200 The law of reciprocity is the key to photographic exposure. It states: the density of the image formed is in direct proportion to the intensity of light and its duration. For a given sensitivity, exposure is the balance between duration (shutter speed) and intensity (lens aperture). To maintain a constant quality of image, duration needs to keep a reciprocal relationship with intensity. If you open the aperture to increase the intensity, then you must reduce the duration. Conversely, if you reduce intensity by making the aperture smaller, duration must be increased to keep the same exposure. Aperture Each step doubles Each step halves more light less light f/1 1000 less time f/1.4 500 f/2 250 f/2.8 125 f/4 60 f/5.6 30 f/8 15 f/11 8 f/16 4 f/22 2 f/32 1 more time Shutter speed Each step halves Each step doubles