Light from any source – a window, a continuously burning lamp, a flash – foltows the same general rule: The light falls off (gets dimmer) the farther the light source is from an object. But light from a flash comes and goes so fast that you can’t see the effect of the flash on a scene at the time you are taking the picture. Special exposure meters are designed for use with flash; you can’t use an ordinary exposure meter to meas. PDF: More About Flash, How to Position It