A rule for estimating exposure without a light meter. On a bright, sunny day, set the aperture to f/16 and the shutter speed to 1 divided by the ISO. For example, at ISO 100 use f/16 and 1/100 second (or the closest speed, 1/125).
For less light, open the aperture: about f/11 for light overcast, f/8 for overcast and f/5.6 for heavy overcast. The rule was printed on many film boxes.
