Sunny-16 rule
From The Glossary of Digital Photography
A rule of thumb used in photography to estimate the proper exposure under sunny conditions when there is no light meter available. The rule is that under midday direct sun, the camera aperture should be set at f/16 and the shutter speed should be set at 1/(ISO speed). So, for example, if the camera were set at ISO 100, the shutter speed would be set at the closest shutter speed to 1/100, which is probably 1/125th of a second.
