In photography, the sunny 16 rule (also known as the sunny f/16 rule) is a method of estimating correct daylight exposures without a light meter. (For lunar photography there is a similar rule known as the looney 11 rule.) 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 f/16 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 f/16 and the shutter speed to 1 or 1 second (on some cameras 1 second is the available setting nearest to 1 second).
- On a sunny day with ISO 200 film / setting and aperture at f/16, set shutter speed to 1 or 1.
- On a sunny day with ISO 400 film / setting and aperture at f/16, set shutter speed to 1 or 1.
As with other light readings, shutter speed can be changed as long as the f-number is altered to compensate, e.g. 1 second at f/11 gives equivalent exposure to 1 second at f/16. More in general, the adjustment is done such that for each step in aperture increase (i.e., decreasing the f-number), the exposure time has to be halved (or equivalently, the shutter speed doubled), and vice versa. This follows the more general rule derived from the mathematical relationship between aperture and exposure time—within reasonable ranges, exposure is 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 and vice versa. Steps in the aperture always correspond to a factor close to the square root of 2, thus the above 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:
Aperture Lighting conditions Shadow detail f/22 Snow/sand Dark with sharp edges f/16 Sunny Distinct f/11 Slight overcast Soft around edges f/8 Overcast Barely visible f/5.6 Heavy overcast No shadows f/4 Open shade/sunset No shadows Add one stop Backlighting n/a