8.2 HOW FLASH WORKS

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Every flash has a maximum useful range. How bright the light from a flash is 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.

Flash light falls off (becomes dimmer) the farther it travels. 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.

When the flash fires, the beam of light expands as it moves father from the camera. 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. 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.

The inverse square law describes the relationship between a subject’s distance and how much light will fall upon it from the flash. If the distance is doubled, only one quarter the amount of light will reach the subject. This is because the light emitted by the flash expands as it moves farther from the camera. The same amount of light is spread over a larger area. Less light therefore falls on a subject of fixed size as the subject moves farther from the flash. 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 middle of the area metered by the autoexposure system. Subjects located farther from the flash will be increasingly darker the farther they are from the flash.