When working with the Canon 580EX II and the new Flash Menu system (available in the Mark III, 40D, XSi and XS), there is a choice in the metering system. When working with these different metering setting is often appears that there is no difference between the two. In fact, in most situations it’s difficult to tell the differences.
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However, the real difference is noticeable when you are using the flash and have no choice but to
shoot into a reflective object. This is when the camera can perform it’s real magic.
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Evaluative meters on the general scene but ignores the brightness of the flash.
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Average Flash Metering will average all the light including that of the flash itself. This can cause for under exposure.
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