sunny 16 rule

the sunny 16 rule is a way to determine a reasonable accurate exposure for pictures in outdoor conditions without consulting a light meter 

it is called the sunny 16 rule because the rule is applied during sunny days and the aperture of the camera is set to f-16 with also the shutter speed to about the  same as your film speed

the f-16 rule is based on how much light falls on the subject.

her is an example:

full sun f-16

half sun f-11

open shade f-8

darker shade f-5.6

darkest shade f-4