Metering. Some times you have a tricky lighting situation or need a convenient "grey card." Use what's around you and meter the sky or the back of your hand, and add one stop - or just meter some grass - to get a roughly 18% exposure. If you don't have a meter, use the "Sunny 16" rule which states that at f/16 you set the shutter speed to the reciprocal of the ISO (e.g. 1/125s for ISO 100). You can adjust from there and if the lighting doesn't change - neither does the exposure. Both film and digital have enough of a latitude to make up small differences.