The Sunny 16 Rule
A simple rule of thumb for taking photos in daylight without a light meter
The Sunny 16 Rule is easy to remember - if you’re taking a photo of a subject under bright daylight set your camera’s aperture to f/16 and shutter speed to the closest reciprocal of the film speed.
f16 + 1/≈ISO
For example, on a sunny day with the aperture set to f/16, using ASA 100 film, you set the camera to the closest available shutter speed; ideally 1/100 but more likely on a rangefinder to be 1/125.
Just in case the sun isn’t shining and you still want to take a well exposed photo without a light meter, here are some variations, still keeping the shutter speed the same as the speed of the film.
f/16 for Sunny
f/11 for Slightly Overcast
f/8 for Overcast
f/5.6 for Heavy Overcast
f/4 for Sunset Sky