Règle de f/16

In photography, the sunny 16 rule (also known as the sunny rule) is a method of estimating correct daylight exposures without a light meter. Apart from the advantage of independence from a light meter, the sunny 16 rule can also aid in achieving correct exposure of difficult subjects. As the rule is based on incident light, rather than reflected light as with most camera light meters, very bright or very dark subjects are compensated for. The rule serves as a mnemonic for the camera settings obtained on a sunny day using the exposure value (EV) system. The basic rule is, "On a sunny day set aperture to and shutter speed to the [reciprocal of the] ISO film speed [or ISO setting] for a subject in direct sunlight." The basic rule is, "On a sunny day set aperture to and shutter speed to the [reciprocal of the] ISO film speed [or ISO setting] for a subject in direct sunlight." For example: On a sunny day and with ISO 100 film / setting in the camera, one sets the aperture to and the shutter speed (i.e. exposure time) to 1/100 or 1/125 seconds (on some cameras 1/125 second is the available setting nearest to 1/100 second). On a sunny day with ISO 200 film / setting and aperture at , set shutter speed to 1/200 or 1/250. On a sunny day with ISO 400 film / setting and aperture at , set shutter speed to 1/400 or 1/500. As with other light readings, shutter speed can be changed as long as the f-number is altered to compensate, e.g. 1/250 second at gives equivalent exposure to 1/125 second at . More in general, the adjustment is done such that for each stop in aperture increase (i.e., decreasing the f-number), the exposure time has to be halved, and vice versa. This follows the more general rule derived from the mathematical relationship between aperture and exposure time—within reasonable ranges, exposure is inversely proportional to the square of the aperture ratio and proportional to exposure time; thus, to maintain a constant level of exposure, a change in aperture by a factor c requires a change in exposure time by a factor 1/c2 and vice versa.