Sunny 16 rule was commonly used back in the days before light meter was invented. It is still useful tool, but nowadays it’s mainly used as a starting point, as a cheap way to check if your camera meter is working or accurate, in case your camera meter is broken and to capture difficult subjects. How to use Sunny 16 rule? First, set your aperture (f-stop) to f/16. Next you need to select your ISO. ISO 100 is commonly used for sunlit scene in outdoors at midday. Finally, set your shutter speed to 1/ISO. If ISO 100 is used, then shutter speed is 1/100. In case your camera doesn’t have 1/100 shutter speed as an option, you can use 1/125 instead. Aperture or shutter speed can be changed to correspond to your needs as long as you compensate by changing the value of the other as well. For example, 1/250 second at f/11 gives equivalent exposure to 1/125 second at f/16.
|Snow / Sand||f/22|
|Open Shade / Sunset||f/4|
|Backlighting||Add one stop|
For night photography there is a similar rule known as the Looney 11 rule.