Sunny 16 Rule
A field estimate stating that in bright, direct sunshine, correct exposure is f/16 with a shutter speed of one over the ISO.
Formula / rule
t = 1/ISO at f/16
Measured in
stops (EV)
In depth
The rule works because sunlight at the Earth's surface is remarkably consistent, sitting near EV 15 for a front-lit subject. It gives a whole family of variants for other conditions — f/11 for light overcast, f/8 for overcast, f/5.6 for heavy cloud, f/4 in open shade — each one stop apart. It remains the fastest sanity check on any meter reading.
Examples in practice
At ISO 200 in full sun, Sunny 16 predicts 1/200 s at f/16, or equivalently 1/800 s at f/8.