Sunny 16 rule
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 obvious 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." Using the rule. 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: As with other light readings, shutter speed can be changed as long as the f-number is altered to compensate, e.g. second at gives equivalent exposure to 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 and vice versa. A change in the aperture of 1 stop always corresponds to  a factor close to the square root of 2, thus the above rule. Alternative rule. An elaborated form of the sunny 16 rule is to set shutter speed nearest to the reciprocal of the ISO film speed / setting and f-number according to this table: Example. This is a scene of average brightness, in direct sunlight. It was shot at ISO 100, f/8 at 1/400 second – the recommended "sunny 16" exposure – which is what autoexposure gave.