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

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Guide Numbers

Guide numbers, usually abbreviated GN, are a way of specifying the power of an electronic flash in a way that can be used to determine the right f/stop to use at a particular shooting distance and ISO setting. In fact, before automatic flash units became prevalent, the GN was actually used to do just that. A GN is usually given as a pair of numbers for both feet and meters that represent the range at ISO 100. For example, the EOS 40D’s built-in flash has a GN of 13/43 (meters/feet) at ISO 100. To calculate the right exposure at that ISO setting, you’d divide the guide number by the distance to arrive at the appropriate f/stop.

Using the 40D’s built-in flash as an example, at ISO 100 with its GN of 43, if you wanted to shoot a subject ...

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