6.3. Ambient light metering versus flash metering

From the point of view of the camera, there are two light sources being used at the same time when a flash unit is fired. Except in total darkness, there will always be some ambient lighting, which is any available light (such as reflected light from the sun or artificial light sources). The camera has no direct control over this lighting. The camera—and you, the photographer—can specify how much of that ambient light hits the film or sensor by adjusting the aperture, shutter speed, and ISO settings.

In the case of automatic flash, the output from the flash unit is fully under the camera's control. The camera can specify when the flash fires and how much light it produces, up to its maximum output.