How Flash Works

The built-in flash pops-up on top of 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.


Autozoom flashheads automatically zoom the flash as you zoom the lens. As you zoom in on a subject, the flash beam narrows, and when you zoom out, it widens. The result is that you have flash coverage of the image area at all times.

The Digital Rebel has a built in pop-up flash and a hot shoe into which you can slip any Canon EX-series flash when you want more power or features. Both options give you access to Canon's advanced flash technology.

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.

When the flash fires, the beam of light expands as it moves father from the camera so it becomes weaker the farther it travels. 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 four times the 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 flash 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.

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