Sunny 16 Rule
The Sunny f/16 Rule is to help us double check our camera to make the proper exposure for a sunny day. Usually on a sunny day, using an aperture of f/16 and a shutter speed that matches the ISO speed will give a good exposure. For example if ISO 200 is used, then use a1/200 second shutter speed.
The f-number of the aperture will need to be adjusted downwards for situations with lower light.
If it isn't sunny then use the following as a guide.
f/11 slight overcast or in the shade.
f/8 overcast some.
f/5.6 heavy overcast with no shadows
f/4 sunrises and sunsets.
Why bother with the Sunny 16 rule when your camera has a built in exposure meter? Well, to double check yourself. Maybe you manually overrode a camera setting and you forgot to change it back later. A quick estimate of the exposure can help assure you the photograph will turn out as you expect. It gives you a manual reference point for proper exposure during the day.