The Sunny 16 rule and its cloudy-day cousins
How one rule of thumb sets daylight exposure, why each weather step opens the aperture one stop, and where it stops working.
The base rule in bright sun
Direct midday sunlight is remarkably consistent in brightness, which is what lets a single rule work. Set the aperture to f/16 and the shutter to one over the ISO, and the exposure lands close to correct with no meter at all. At ISO 100 that pairing is f/16 and 1/100 second. Photographers relied on this for decades, and film boxes still print exposure guides based on it.
A stop for each kind of weather
As clouds thicken, less light reaches the scene, so the rule opens the aperture one stop at a time to compensate. Slight overcast moves to f/11, overcast to f/8, heavy overcast to f/5.6 and open shade to f/4, each a one-stop jump that doubles the light let in. The shutter can stay put because the aperture is doing the compensating. Stepping through the presets in the tool shows this ladder directly.
Matching real camera shutter speeds
Cameras offer a fixed set of shutter speeds, usually 1/60, 1/125, 1/250 and so on, rather than an exact 1/ISO. In practice you round the Sunny 16 shutter to the nearest marked speed and accept the tiny difference, which is well within exposure latitude. If you prefer to keep a specific shutter for motion, this calculator lets you instead solve for the aperture that pairs with it. Either way the goal is the same total exposure.
When the rule misleads
Sunny 16 assumes front-lit subjects under open sky in strong daylight. Backlighting, deep shade, snow and water throw it off because the effective brightness on your subject differs from the open-sky assumption. Sunrise and sunset light is far dimmer and warmer than midday sun, so the rule overexposes there. Treat it as a dependable baseline for ordinary daylight and reach for a meter or bracketing in the tricky cases.