Sunny 16 rule for film exposure
To correctly expose an object lit by bright sun, use a shutter speed that is as close as you can get to the film speed and an aperture of f/16. That’s it. Simple! And effective.
As an example, if you are using film with a speed of ISO 400, set your camera’s shutter speed for 1/400 sec at ƒ/16 to shoot a subject in bright sun. If you do not have a shutter speed of 1/400 sec, use the closest shutter speed, which is probably 1/500 sec. To be sure of at least one correct exposure, bracket on either side of 1/400 second – that is, take one shot at 1/250 sec at ƒ/16 and another at 1/500 sec at ƒ/16.
Similarly, if your film is ISO 100, use the closest shutter speed setting, which is 1/125 sec at ƒ/16, and then shoot an insurance shot at 1/60 sec at ƒ/16.
BUT HOW DO YOU USE THE RULE WHEN YOU DON’T WANT TO SHOOT AT ƒ/16?
That’s easy. Let us say you want to shoot at ƒ/8. Simply use the base settings as a starting point, say they are 1/125 sec at ƒ/16 because you are using ISO 100 film, and then figure out what the shutter speed would have to be if you change to ƒ/8. How do you do that? Well, ƒ/8 is two stops wider (more open) than ƒ/16, which means the lens will let in four times the amount of light than it will at ƒ/16, so increase your shutter speed by two stops to 1/500 sec to compensate and keep the exposure the same.
The sunlight must be bright, with little haze. If you’re shooting on sand or snow, the additional reflected brightness requires you to choose either a faster shutter speed or a smaller aperture.
IF THE SUBJECT’S NOT IN BRIGHT SUNLIGHT, WHAT GOOD IS THE SUNNY 16 RULE?
It’s still a helpful guideline, but requires some guesswork on your part. If the sky is overcast, you have to estimate how much of a difference the reduced light will make. This is where experience comes in. If you figure there would be a one stop difference between a normally bright, sunlit scene and an overcast scene, then increase your aperture opening by one stop (i.e go from ƒ/16 to ƒ/11) or decrease your shutter speed by one stop (1/125 sec to 1/60 sec). It sounds tricky, and it is a bit because guesswork is involved, but as you shoot more and more pictures, you will find that your guesswork becomes more refined, and you can usually estimate an acceptable exposure.
If your subject is backlit or sidelit, you may also have to make an exposure compensation. Keep in mind your subject’s tonal brightness. A white or light-colored subject will need less exposure than a dark one.
Here are some guidelines for exposure adjustment for varying conditions:
– Stop down (decrease exposure) one to two stops for snow or sand (depending on how reflective surrounding surfaces are);
– Increase exposure by one stop when conditions are lightly overcast (when there is thin cloud at high altitude);
– Increase exposure by two stops when it is overcast but bright (shadows cannot be seen), and for subjects that are backlit;
– Increase exposure by three stops in open shade (i.e. a shadowed area on a bright day);
– Increase exposure by three to four stops when conditions are heavily overcast.