Sunny 16 rule

In photography, the sunny 16 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 system.

Using the rule

The basic rule is, "On a sunny day set aperture to and shutter speed to the ISO film speed for a subject in direct sunlight." In simplest terms, bright sun = f:16 @ 1/film-speed-number.
For example:

On a sunny day at ISO 100, the aperture is set to and the shutter speed to or seconds.
On a sunny day at ISO 200 and aperture at, set shutter speed to or.
On a sunny day at ISO 400 and aperture at, set shutter speed to or.

Shutter speeds can be changed as long as the f-number is adjusted accordingly, e.g. second at gives equivalent exposure to second at. Exposure adjustments are done in a manner that retains the EV. As the aperture is opened the shutter-speed/exposure-time is reduced by a factor of approximately one-half This follows the mathematical relationship between aperture and shutter speed where 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

The sunny 16 rule can be used in varying light by setting the shutter speed nearest to the ISO film speed and f-number according to a generalized exposure table, as: