Before digital cameras had built-in metering, it was up to the photographer to determine the correct exposure for a scene. Back in the days of film photography you had to bring a light meter and other expensive gear just to figure out the exposure. Because of this, photographers came up with rules to help them estimate exposure. The sunny 16 rule is just that – a simple way to estimate the correct exposure during daylight.
Although, modern cameras have very capable metering technology built-in, the sunny 16 rule and other similar rules still apply today. Being able to evaluate exposure without the camera’s help can make you a more confident photographer and it can also help when you get into some more difficult lighting situations.
How does it work?
The sunny 16 rule states that in bright sunny conditions if you set the aperture to f/16, the exposure will be correct if the shutter speed is the inverse of the ISO. That sounds more complicated than it really is, so here’s an example:
If you’re shooting outside and it’s sunny, set the aperture to f/16. If your ISO is set to 100, then the shutter speed should be 1/100 or 1/125 (the inverse of the ISO). If you change the ISO to 400, then the shutter speed should change to 1/400 as well. It’s that simple.
The reason this works is when you increase the ISO, the sensitivity increases and the image becomes brighter. To compensate and keep the same exposure, you have to use a faster shutter speed. Whenever you increase aperture, shutter speed, or ISO, you must decrease one of the other two an equal amount in order to keep the same exposure.
As with any rule in photography, it’s more of a guideline. The sunny 16 rule is a good place to start, but you can change the aperture to anything you want, as long as you adjust the ISO and shutter speed to compensate.
Other lighting situations
What about when it’s not sunny outside? There are other rules for other types of light, they just aren’t as well known. The chart below has guidelines for shooting in all types of outdoor light with some basic ISO and shutter speed combinations. Feel free to experiment with these to find out what works for your shot.