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April 27, 2009

Comments

Sweet. That "Beyond the Instruction Manual" article on Canon's website was very instructive!

Those fine details about the way FEC/AEC/FEL behave clarified some things I had never been absolutely sure about...

Nice! I had known about FEL/FEC behavior from piecing together various articles on the Speedlite system for years, but thanks for putting it all in one convenient article!

Hmm... Have you read Chuck Westfall's Tech Tips for May 2009? There was a question this month that went like this:

"I often use my EOS-1D Mark II in Av mode. If I dial in exposure compensation will my auto flash meter differently?"

After reading the second part of your article on the Canon flash system, I was not expecting this answer from Westfall:

"In the EOS system, flash exposure is measured and controlled independently from ambient exposure. Therefore, the level of flash exposure is not affected when you dial in ambient exposure compensation."

This seems to contradict what I read in your article... Would you care to comment/add to what you said? I'd like to get to the bottom of this! ;)

Thanks.

"In the EOS system, flash exposure is measured and controlled independently from ambient exposure. Therefore, the level of flash exposure is not affected when you dial in ambient exposure compensation."

It's actually a bit more complicated than that--as if it weren't already complicated enough! What happens depends on what exposure mode you're in.

1. If you're in Av mode, adjusting ambient exposure compensation changes only the shutter speed. When dialing in negative compensation (less exposure) you therefore have to be careful that the shutter speed doesn't go higher than the flash sync speed. The shutter speed indicator in the viewfinder will starting flashing on and off if this happens. If you dial in positive compensation (more exposure) the shutter will be open longer. Depending on how much positive compensation you dial in and the amount of ambient light, this could result in overexposure for flash and ambient.

2. If you're in Tv mode, adjusting ambient exposure compensation changes only the aperture. As long as the camera is set at or below the correct sync speed, the flash will automatically adjust its exposure to the change in aperture, therefore ambient exposure compensation has little effect on the flash exposure.

3. If you're in P mode, the camera automatically the shutter speed AND the aperture. When the flash is attached on on, the camera automatically keeps the shutter speed at or below the correct speed for flash sync. Adjusting ambient exposure compensation therefore changes EITHER the aperture OR the shutter speed, depending on the ambient light level AND whether the flash is attached and on. A flash+ambient exposure may therefore get lighter or darker, depending on the ambient light level and the amount of ambient exposure compensation you dial in.

So in short, Chuck is right and I was wrong. In my defense, before I wrote the article I gave the ambient vs. flash exposure compensation a quick test in P mode. When I dialed in -1 stop of ambient exposure compensation with flash attached and on, the overall exposure got darker. The reverse happened when I dialed in +1 stop of ambient exposure compensation. I therefore assumed--incorrectly--that the same would happen in the other modes as well, regardless of the ambient light level. Let me also add that the article was reviewed before publication by one of Canon's flash experts and that the paragraph on ambient exposure compensation section passed without comment.

Thank you for pointing out the error. I'll forward a copy of this response to Canon and request that the article be corrected ASAP.

Sigh. It's no wonder so many photographers are afraid to use flash.

From your initial flash article, the way I had understood it is that if I dialed in -1 in AEC, I would have to dial in +1 in FEC to bring the flash back to normal power (therefore having the ambient light a stop darker, making the subject stand out from the background by a stop). But if I understand correctly what you and Mr. Westfall are saying now, then all I would have to do to achieve this effect is to dial in -1 AEC!

This is too bad, because I thought that the other way, though more complex, actually made more sense!

Thanks for the precision! I admire your honesty regarding this little mistake.

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