Wikipedia: 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. 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.
- 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 1/100 or 1/125 second (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 f/16, set shutter speed to 1/200 or 1/250.
- On a sunny day with ISO 400 film / setting and aperture at f/16, set shutter speed to 1/400 or 1/500.
Cousin David illustrates the “Sunny 16” in this image. It works! The sky is a little blown out. But there is a serviceable image. Back in the day when ISO was fixed to the film in the camera, this rule was in the Kodak paper enclosed in every box of film and on the box itself. Many cameras did not have a meter. Many more did not have changeable settings. Where you could set shutter speed and f-stop, this rule applied and got something on the negative, which could be printed. Of course this was a rough approximation. However anything that could be printed was a plus. Most amateur photographers at the time were largely unsuccessful so that this rule helped.