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Canon EOS 40D Guide to Digital Photography by David D. Busch

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How Electronic Flash Works

The bursts of light we call electronic flash are produced by a flash of photons generated by an electrical charge that is accumulated in a component called a capacitor and then directed through a glass tube containing xenon gas, which absorbs the energy and emits the brief flash. For the pop-up flash built into the EOS 40D, the full burst of light lasts about 1/1,000th of a second and provides enough illumination to shoot a subject 10 feet away at f/4 using the ISO 100 setting. In a more typical situation, you’d use ISO 200, f/5.6 to f/8 and photograph something 8 to 10 feet away. As you can see, the built-in flash is somewhat limited in range; you’ll see why external flash units are often a good idea later in this ...

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