The sunny 16 (and overcast 8) rule is an easy way to remember what the values are, and it's not really an average: The incident light on a scene on a cloudless

The sunny 16 (and overcast 8) rule is an easy way to remember what the values are, and it's not really an average: The incident light on a scene on a cloudless day when the sun is directly overhead doesn't change. The average reflectance of the scene changes, but in photography you mostly don't care about the average reflectance, you care about the incident light.