Expositor and Expositor Lite are handy slide-rule based exposure calculators for photography.

Pick the situation you're shooting, then slide the ruler until your camera's ISO is lined up next to it, and every combination of F-stop and shutter speed on the other side will get you close to a perfect exposure.

Use it in the field if you don't have a light meter handy, or use it to train your brain to be its own exposure calculator. Expositor is here to help you shoot great pictures!