How Automatic Flash Works
Click to explore the inverse square law.
The power of a flash is indicated by its guide number. Click here for an Excel worksheet you
can use to explore these numbers.
Since flash falls off with distance, objects near the flash will be lighter in a picture than objects farther away. You can use this to advantage;
for example, at night you can isolate a subject against a dark background.
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.
The light from a flash falls off with distance. When you double the distance, you get onequarter as much light. This relationship is
called the inverse square law.
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 autoexposure system. Subjects located farther from the flash will be increasingly darker the farther they are from the flash.