Moonrise & Aurora

Moonrise & Aurora

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up vote 10 down vote favorite

Taking photos with my Canon XTi and 550ex. Camera exposure compensation did not seem to effect the exposures. I figured out I needed to use the on-flash exposure comp.

But the flash output is controlled by the camera through TTL, why did the camera not boost the flash output when I asked THE CAMERA for increased exposure ?

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2  
Can someone answering clarify how the regular exposure compensation operates when flash is in use? If you're not using slow sync, is it effectively meaningless? If you are using slow sync, does it affect the length of the shutter but not the flash impulse? If you simply dial down the flash EC by two stops and the "regular" EV +/- up by two, do you get a balanced exposure with less flash than the default? In slow sync mode or always? Does it matter if the camera is in Tv mode, or can it also be in Av or P? – mattdm Jan 13 '11 at 1:58

2 Answers

up vote 6 down vote accepted

On an intuitive level, you might think that exposure is exposure, and even though there are two different metering systems involved, they should talk to one another. That would be perfect if exposure was always either ambient or flash, but it stops short when you consider that flash is just as often used to augment ambient light, either to fill in shadows or to light a foreground subject in sharp relief against a darker background. Having the two systems work independently is a lot more useful in a lot more situations -- you may want to bracket the flash while leaving the ambient steady, or bracket the ambient with a constant flash, and so on. It does mean remembering that they're separate systems, but it really is better that way.

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+1, having flash and regular exposure control tied together would take half of the fun away. – che Jan 13 '11 at 6:11
So did increasing camera exp comp. allow in more ambient and reduce the flash output slightly to give the same exposure? – Ken Jan 13 '11 at 9:05
No -- it's just that indoors, unless the space is relatively large, ambient light contributes negligibly to the overall exposure in most cases. If it were contributing, say, ten percent of the total, then doubling the amount of ambient light would not appreciably change the overall exposure. The increase would be drowned out by flash reflection. And if you on aperture priority with a flash-only setting, the shutter speed likely wouldn't change from the X-sync speed (in order to avoid unwanted artifacts, like ghosting and trails), depending on the camera. – Stan Rogers Jan 20 '11 at 19:06
up vote 3 down vote

The Canon XTi has something called FEC - Flash Exposure Compensation - that's what controls flash power when using E-TTL. Normal EC is for metering ambient light.

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