The Sunny 16 rule is a rough rule of thumb that was used back in the film camera days. Way before all of the fancy digital technology that’s simplified the calculations needed. But to better understand how light metering works, it’s helpful to have a look at the Sunny 16 rule.
Sunny 16 rule explained
The name is exactly what we’d expect. On a bright and sunny day, I can use f/16 as an aperture to get proper exposure with. However, when dealing with exposure, I have to also consider my ISO value, which is the light sensitivity of my film. And also my aperture value, which is how wide open or closed the hole that lets in light is.
The meat of the rule is that at f/16, my shutter speed has to be the reciprocal value of the ISO of my film. So if I’m using ISO 200 because it’s a bright and sunny day, then I should be shooting at 1/200ths of a second for proper exposure. If I raise my shutter speed to 1/500ths of a second, the image will likely be too dark. And if I were to lower it to 1/50ths of a second, the resulting image will probably be too bright.
And if I’m using film of a different sensitivity, like ISO 400, the shutter speed is again the reciprocal value, which is 1/400ths of a second at f/16.
Photo by Frantisek_Krejci / CC0 1.0
So what if the weather isn’t bright and sunny? All we have to do is bring our aperture value down a stop of light. Each stop doubles the amount of light entering the camera. f/11 is twice as wide as f/16. And f/8 is twice as wide as f/11, or four times as wide as f/16.
I might use f/11 on a slightly cloudy day. While if it’s starting to thunderstorm and get even darker, I’ll switch to f/8 to ensure my film has enough light to work with.
Photo by photo-graphe / CC0 1.0
Depth of field
But I also have to remember that as I adjust the aperture the depth of field is also changed. Depth of field is how much of the scene is in sharp focus. Say I’m shooting a landscape of a wide valley with far off mountains on a bright sunny day. I might instead want to use f/22.
If I do and I’m using ISO 100 film, the Sunny 16 rule suggests I use a shutter speed of 1/50ths of a second instead. Why? Because f/22 only allows half as much light into the camera as f/16 would have. So I need a slower shutter speed to compensate for the loss of light. Make sense?
The Sunny 16 rule is not nearly so necessary anymore, as modern interchangeable lens cameras have plenty of tools to ensure proper exposure. But there are many beginners looking to get further into the guts of photography. And film photography is far from dead. Thus, the Sunny 16 Rule remains a great place to learn more about photography!