And since dedicated cameras measure light at or near the film plane, you can use the flash in a bounce or off-camera mode and still get good exposure automatically. In any flash situation, the camera may adjust flash power or aperture based on distance information provided by an autofocus lens. In extreme close-up photography and outdoor flash fill photography, for example, you may be faced with situations in which manual flash exposure would provide more accurate and predictable results. To determine the correct lens opening for proper exposure with a manual flash unit, you need to know the light output of the electronic flash unit you're using, the speed of the film you're using, and your distance from your subject and how to calculate these factors. If your flash unit doesn't have a calculator dial, you need to use a guide number for your calculations.
Guide numbers are easy to use and are usually provided in the flash manual.
The result is the lens opening to use.