The Sunny f/16 Rule: What It Is and When It Breaks Down
The sunny 16 rule is one of the oldest in photography and one of the most useful. It gives you a correct exposure in bright sun without a meter. It is also often misapplied, and it breaks down in a few specific situations worth knowing in advance.
The rule
In bright, direct sunlight, set the aperture to f/16 and the shutter speed to the reciprocal of the ISO. At ISO 100 that is f/16 at 1/100 s, at ISO 200 f/16 at 1/200 s, and at ISO 400 f/16 at 1/400 s. Any equivalent works too: f/16 at 1/100 s is the same exposure as f/8 at 1/400 s or f/4 at 1/1600 s.
It works because clear-sky sunlight is very consistent. Full sun delivers on the order of 100,000 lux to a horizontal surface once the sun is more than about 30 degrees above the horizon, whether you are in Iceland or the Sahara. For a mid-toned, front-lit subject that corresponds to exposure value 15 at ISO 100, which is exactly f/16 at 1/100 s.
The rest of the family
Each step down in lighting costs one stop. Keep the shutter at 1/ISO and open the aperture:
| Light | Aperture at 1/ISO | Stops darker than sunny |
|---|---|---|
| Bright sun, hard shadows | f/16 | 0 |
| Slight haze, soft shadows | f/11 | 1 |
| Overcast, no shadows | f/8 | 2 |
| Heavy overcast | f/5.6 | 3 |
| Open shade, or sunset | f/4 | 4 |
So overcast is two stops darker than sunny, not three. At ISO 100, an overcast exposure is f/8 at 1/100 s, or equivalently f/4 at 1/400 s.
Using it with any aperture
You do not have to shoot at f/16. Count stops from f/16: f/11, f/8, f/5.6, f/4, f/2.8, f/2, f/1.4. That is seven stops from f/16 to f/1.4, five to f/2.8 and three to f/5.6. Each stop wider means a shutter speed twice as fast. At ISO 100 in full sun:
| Aperture | Sunny | Overcast |
|---|---|---|
| f/1.4 | 1/13000 s | 1/3200 s |
| f/2.8 | 1/3200 s | 1/800 s |
| f/5.6 | 1/800 s | 1/200 s |
| f/8 | 1/400 s | 1/100 s |
That tells you at a glance what a shallow-focus plan costs. For a shallow-focus portrait at f/1.4 in bright sun at ISO 100, you need about 1/13000 s, beyond the 1/8000 s limit of most mechanical shutters. You can add a neutral density filter, three stops of which brings it to about 1/1600 s, use a faster electronic shutter if your camera offers one, or settle for f/2.8 at 1/3200 s.
To freeze action on an overcast day at ISO 400, the rule gives f/8 at 1/400 s. To get 1/1000 s you need about 1.3 stops more light, so open to around f/5, or keep f/8 and raise the ISO to 1600.
When it breaks down
Low sun. The rule assumes the sun is well above the horizon. As it drops below about 10 degrees, the atmosphere absorbs more and light falls off by a stop or two. An hour before sunset the rule starts to drift, and within half an hour of sunset it is well off. Open a stop in the golden hour and two in the last 15 minutes.
Backlit subjects. The rule describes a front-lit subject. If the sun is behind the subject, the side facing you is in shade, which needs about three stops more than sunny 16, like open shade. Applying the plain rule gives a silhouette. You can expose for the subject and let the sky blow out, expose for the sky and silhouette the subject, or fill with flash.
Snow and sand. Here the rule has an advantage over the meter. It is based on the light falling on the scene, not on what the scene reflects, so it handles bright surfaces correctly, while a camera meter tries to render snow as mid-grey and underexposes by about a stop. If you use the meter in snow, add a stop of positive compensation, and in either case check your highlights.
Latitude and altitude. At high altitude there is a little less air above you and the light is slightly brighter, by a fraction of a stop. Near the poles the sun never gets as high. In practice the rule works well from the tropics to about 60 degrees of latitude.
Still useful today
Modern meters do the job, but the rule stays useful in several ways: for film cameras with no meter, for checking that your meter is not being fooled, for presetting exposure when you do not want to fiddle with settings, and for judging the exposure cost of depth of field choices without looking at the camera. It also gives you "Looney 11" for the Moon: because the Moon is lit by the Sun, f/11 at 1/ISO works at full moon. The astrophotography post covers the rest of night-sky exposure.
To see what depth of field your chosen aperture gives, use the calculator.
Related reading: Diffraction: Why Stopping Down Too Far Hurts Sharpness, Aperture vs Distance: What Affects DOF More?, Zone Focusing: The Street Photographer's Secret Weapon.