Sunny 16 rule

Apart from the advantage of independence from a light meter, the sunny 16 rule can also aid in achieving correct exposure of difficult subjects.

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: Shutter speeds can be changed as long as the f-number is adjusted accordingly, e.g. ⁠1/250⁠ second at f/11 gives equivalent exposure to ⁠1/125⁠ second at f/16.

A change in the aperture of 1 stop always corresponds to a factor close to the square root of 2, thus the above rule.

Sunlit subject shot on a digital camera set to ISO 100, exposed at f/8 at 1/400 second which is the same exposure value as f/16 for 1/100 second, the recommended "sunny 16" exposure
Tessina with exposure guide plate from the 1960s. At that time, DIN 21 was equivalent to ASA 80. After 1983, DIN 21 was ASA 100. [ 5 ] On this guide plate, DIN 21 uses f/16 and 1/125, consistent with Sunny 16.