It's simple: Sunny 16 says that on a sunny day, you should set your aperture to F16 and your shutter speed to the reciprocal of your film speed, or as close to it as you can get. So if you're shooting 100 ASA film, set F16 and 1/90 or 1/125 sec. Running 400 speed film? Try F16 and 1/500 sec.