The Sunny 16 rule is a method for setting a correct exposure in bright sunlight without using a light meter. It states that on a clear sunny day, with the aperture set to f/16, the correct shutter speed is the reciprocal of the ISO. At ISO 100 that means roughly 1/100 or 1/125 second, at ISO 200 about 1/200, and so on.
The rule works because direct overhead sunlight is remarkably consistent in intensity, so a fixed combination of settings reliably produces a good exposure. It gave film photographers a dependable baseline before in-camera metering, and it remains valuable today as a sanity check when a meter might be fooled by a very bright or very dark scene, and as the foundation for shooting fully manual film cameras that have no meter at all.
Because exposure settings trade off against one another, you are not locked to f/16. The same exposure can be reached with equivalent combinations: opening up to f/8 and shortening the shutter speed by two stops gives the identical brightness, which lets you choose the depth of field you want while keeping the exposure correct. Understanding this links Sunny 16 directly to the exposure triangle.
The rule extends to other conditions by opening the aperture as light decreases, keeping the shutter at one over the ISO. Slight overcast or hazy sun calls for f/11, overcast for f/8, heavy overcast for f/5.6, and open shade or sunset for f/4, each one stop brighter than the last. Subjects that reflect extra light, such as snow or bright sand, call for closing down to f/22.
A worked example makes it concrete. On a sunny afternoon at ISO 100, you start at f/16 and 1/125 second. Move into the shade of a building and the scene drops roughly three stops, so you open to f/5.6 at the same shutter speed, or keep f/16 and slow to about 1/15, which would then need a tripod. The rule gives you a starting point, and a glance at the histogram or a chimp at the screen confirms it.
Sunny 16 is also a teaching tool, because it builds an intuition for how much light a scene carries and how the f-stop and shutter relate. Photographers who can estimate exposure this way are rarely caught out by a misleading meter reading, and they understand why a meter aiming for middle gray sometimes needs to be overridden. Practiced enough, it lets you shoot confidently in manual mode with only an occasional glance at the result.
The rule is worth committing to memory even if you always shoot with a meter, because it gives you an independent check. If your meter suggests something wildly different from what Sunny 16 predicts for the conditions, that is a signal to look closer, since the meter may be reading a very bright or very dark area and need overriding.