How Automatic Flash Works

Every flash has a maximum useful range.
The intensity of the flash when it reaches a subject depends on the flash's power and on how far the light has to travel.

The further the subject is from the flash, the less light will reach it and so the less light will be reflected from the subject back toward the camera.



When the flash fires, the beam of light expands as it moves father from the camera so its intensity falls off with distance.

As a result, subjects nearer the flash will be illuminated with a more intense light than subjects farther away. The rate at which the light falls off is described by the inverse square law. The law states that if the distance between the flash and subject is doubled, only one quarter the amount of light will reach the subject because the same amount of light is spread over a larger area. Conversely, when the distance is halved, four times as much light falls on a given area.

When subjects in an image are located at different distances from the camera, the exposure will only be correct for those at one distance—normally those closest to the camera or in the area metered by the auto-exposure system. Subjects located farther from the flash will be increasingly darker the farther they are from the flash

David B Flynn.