by Rodrigo

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Where you're getting confused is in how shutter speed affects flash exposure vs. ambient exposure. If you typically don't use a flash, you assume that a longer shutter speed will create a brighter exposure, no matter what. In most situations, a flash burst is many times faster than your shutter speed. Keeping the shutter open for a longer period of time ...


In Av (and Tv) mode, flash is not assumed to be primary light source, so camera will choose exposure to match metered ambient light. In P mode, however, the camera tries to ensure exposure time is quick enough for handheld shooting, and thus will happily expose for the flash-illuminated subject, ignoring the lack of ambient light. To put it in flash terms, ...


Canon DSLR's operate in so-called "slow sync" mode when in Av mode. That means, the exposure time is set to expose for ambient light, just like without flash. This is useful to freeze your foreground subject with the flash, but still get a natural background lighting. (try shooting a person at night in the open to see the difference!) I like to dial in a ...

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