The Sunny 16 Rule is a way to meter for correct exposure during daylight without using the camera’s meter.
The basic rule of thumb is that if you have a clear, sunny day and your aperture is at f/16, whatever ISO you are using, your shutter speed will be the reciprocal value of that ISO value
So for example, if your ISO is 200 at f/16, then your shutter speed will be 1/200 seconds. If your ISO is 100, then your shutter speed will be 1/100 seconds.
The Sunny 16 Rule is a good way to check if your camera is spot on with exposure or does it consistently under or over expose. Some cameras have a tendency to slightly under expose, and this is a good way to test your camera.
Additionally, unlike the camera metering system, the Sunny 16 Rule is based on incident light instead of reflected light, which means that it’s based on the brightness of the light only, and not how the light that is being reflected off the subject and into the camera.
Go ahead… try it and see how your camera performs…