A handheld or in-camera meter can fail in ways that are easy to miss: a flat battery, a backlit frame that fools a reflected reading, or a scene dominated by snow or dark foliage. The Sunny 16 rule provides an exposure estimate that depends on nothing but the weather, which makes it both a fallback when no meter is available and a way to confirm that a metered value is plausible.

The base rule

The rule states that on a clear, sunny day a frontlit subject is correctly exposed at f/16 with a shutter speed equal to the reciprocal of the film’s ISO speed. ISO 100 film calls for 1/100 second at f/16; ISO 400 film calls for roughly 1/400 second at f/16. Because exposure is a product of aperture area and time, any equivalent combination holds the same total light: f/11 at 1/250, or f/8 at 1/500, deliver the same exposure as f/16 at 1/125 for ISO 125 film. The shutter speed and aperture trade against each other in whole stops while the ISO anchors the starting point.

The rule is built around incident light, the light falling on the subject, rather than the reflected light an in-camera meter reads. Direct midday sun is consistent enough in intensity that a fixed setting works. Because it ignores subject tone, the estimate is not pulled off by a very bright or very dark subject the way a reflected-light meter can be.

Adjusting for cloud and shade

The rule scales by opening the aperture one stop at a time as the light softens, judged by the shadows the subject casts. Wikipedia’s summary of the rule pairs each step with a shadow cue: distinct shadows at f/16 in full sun; soft-edged shadows at f/11 under slight overcast; barely visible shadows at f/8 under overcast; no shadows at f/5.6 in heavy overcast; and open shade or sunset at f/4. Each stop doubles the exposure, compensating for the falling intensity while the shutter speed and ISO stay fixed.

Using it as a sanity check

Held against a meter, the rule flags gross errors. A bright frontlit scene that meters far from the Sunny 16 value usually signals a fooled reflected reading, a wrong ISO setting, or operator error rather than a genuine exposure. As a back-of-envelope reference it is accurate to within about a stop, which black-and-white negative film, with its wide exposure latitude, absorbs comfortably.