What happens when you are using a film camera such as a Pentax K1000 and the battery that is powering your light meter dies? Or maybe you are using a camera that doesn't have a light meter at all such as a Nikon F or a Leica M2. You might have an external light meter such as the hot shoe mounted KEKS light meter. If you don't, then using sunny-16 will be your best bet at ensuring your photos are correctly exposed.
How do you use the sunny-16 rule?
First you want to set your shutter speed to the closest number to the ISO of the film you are using. For example if you are using Kodak Portra 160 which has an ISO of 160 then you want to set your shutter to 125 (1/125).
Slower is better!
It is better to round down to the closest shutter speed because film better handles being overexposed than being underexposed. For example if the film you are using has an ISO of 200 then 1/250 is the closest shutter speed, but it would be better to go for 1/125. See the table for reference:
The next step is to set the aperture of your lens according to the light levels. As the name suggests when it is sunny you should set your aperture to f/16. As the light levels decrease you need to increase the size of your aperture (decrease the number). See the table below for reference:
|Cloudy with some sun
|Cloudy but still fairly light
|Sunset or sunrise
Reading the light levels can take a bit of practice, it can be useful to note down what settings you used for each shot so you know how to improve for next time.