Basically it means mixing flash and ambient light. Typically you'll need to 'drag the shutter' by setting a long shutter speed to capture the ambient light, but have a very short burst of flash.
Essentially when you do this, you are taking two exposures in one.
There is the flash exposure - which you need to set your flash for and the aperture for - but shutter speed is largely irrelevant once it is fast enough, as the shutter time is always much larger than the flash pulse time.
The second exposure is the ambient exposure - you've set the aperture for the flash, so now you need to set the shutter for the ambient light to balance it correctly.
Using this method you can play around with different intensity in the sky compared to the subject and so on.
It is also often used at weddings to capture a 'stop action' with motion shot of a bride/ groom dancing, etc.
On pretty much any of the canon bodies, the Tv and Av modes do this automatically with an E-TTL flash connected. The exposure calculated in
these modes have nothing whatsoever to do with the flash output, it is all ambient exposure readings. Dunno how the drebel handles it though, I suspect just the same. It is a lot easier to manage with a controllable flash (i.e., not a 420ex) and in manual mode, where you can meter the ambient light, establish the settings you need and off you go.
E-TTL flash and general through the lens metering complicate things as you don't know shot to shot how the camera will have changed things. This is particularly out of control in a matrix metering mode.
see here for a long discussion
Message edited by author 2004-10-08 14:59:04.
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