A term from the pre-digital, film days, Totally Rad came up with a handy, dandy infographic with tips on how to shoot outdoors on a clear, sunny day without a light meter.
Wikipedia explains the rule:
The basic rule is, "On a sunny day set aperture to f/16 and shutter speed to the [reciprocal of the] ISO film speed [or ISO setting] for a subject in direct sunlight." For example:Or, for other lighting conditions, Wikipedia has this great little graphic.
On a sunny day and with ISO 100 film / setting in the camera, one sets the aperture to f/16 and the shutter speed to 1/100 or 1/125 second (on some cameras 1/125 second is the available setting nearest to 1/100 second).
On a sunny day with ISO 200 film / setting and aperture at f/16, set shutter speed to 1/200 or 1/250.
On a sunny day with ISO 400 film / setting and aperture at f/16, set shutter speed to 1/400 or 1/500.
|Aperture||Lighting Conditions||Shadow Detail|
|f/22||Snow/Sand||Dark with sharp edges|
|f/11||Slight Overcast||Soft around edges|
|f/5.6||Heavy Overcast||No shadows|
|f/4||Open Shade/Sunset||No shadows|
|Add One Stop||Backlighting||n/a|