The Sunny 16 Rule, Explained

The rule in one sentence

On a sunny day, at f/16, your shutter speed should equal the reciprocal of your film ISO. ISO 100 film at f/16 gets you 1/100 second. ISO 400 at f/16 gets you 1/400. That is the whole rule.

Everything else is equivalent exposures. Open up to f/11 and you need 1/200 at ISO 100. Open up to f/8 and you need 1/400. Stop down to f/22 and you need 1/50. The aperture and shutter trade off in stops, one for one.

Where the rule comes from

Sunlight at noon on a clear day on a temperate latitude delivers about 100,000 lux to a horizontal surface. Reflected off an average scene (about 18 percent reflectance, which is what light meters assume), that becomes roughly 18,000 lux off the subject. The exposure equation N² over t = ES × L / K (where N is f-number, t is time, S is ISO, L is luminance, and K is the meter calibration constant) resolves to f/16 at 1/ISO for those conditions.

That is not magic. It is what the daylight delivers, what 18 percent grey reflects, and what film responds to. The match is good enough that Kodak printed the rule on the back of every film box for decades.

The full Sunny 16 lookup table

Sunny 16 generalises to other lighting conditions by opening the aperture one stop at a time (or changing shutter speed equivalently). At ISO 100:

For ISO 400 film, every exposure shortens by two stops, so the same scene at f/16 wants 1/400 second. For ISO 50 (like Velvia), every exposure lengthens by one stop, so f/16 wants 1/50 second.

The five conditions where Sunny 16 fails

1. Snow and beach scenes. The sand or snow reflects two or three times what a meter assumes, so Sunny 16 over-exposes dramatically. Add a stop or two (f/22 instead of f/16 at noon on fresh snow with sun).
2. Backlit subjects. Your subject is in shadow against a bright background. Sunny 16 reads the background and under-exposes the subject by several stops. Open up two to three stops, or spot-meter the subject directly.
3. Dawn, dusk, and golden hour. Light intensity changes by a stop every few minutes. Sunny 16 is a snapshot of one specific lighting condition. Use a meter for anything that is not midday.
4. Mixed light. Window light, fluorescent, tungsten, fire, mixed colour temperatures. Sunny 16 assumes daylight on a surface; mixed light breaks that assumption.
5. Anything indoors. Even bright indoor light is typically four to eight stops darker than Sunny 16 expects. The rule does not apply inside.

The right way to use Sunny 16 in 2026

As a sanity check. If your meter reading disagrees with Sunny 16 by more than a stop on a sunny day in a normal scene, one of them is wrong, and it is usually the meter (battery, calibration, pointed at a bright sky, pointed at a dark hat). Sunny 16 catches gross meter errors before they become unprintable negatives.

As a starting point if your meter is dead or your subject is too backlit for confidence. Apply the table, then bias for the scene (open a stop for snow, open two for backlit, etc.).

As insurance for slide film. Slide film has no latitude. If Sunny 16 and the meter disagree, take both exposures with a bracket.

Where Zone Light Meter helps

The app shows the Sunny 16 prediction next to the metered reading whenever the conditions are recognisably outdoor. If the two disagree by more than half a stop, the app flags it and explains the likely reason (snow scene, backlit subject, deep shade) so you can decide which to trust.

For practiced shooters who like Sunny 16 as a first instinct, the app's manual mode lets you skip the meter entirely and choose exposure from the lookup table. The shot log records both the method you used and the result.