This blog is my attempt to keep track of some of the things I learn along the way, with my Canon 7D, G12 and accessories. All images copyright Brad Calkins, not to be used without permission (or purchase). I do not attempt to monetize my blog, other than to promote my stock photo portfolio on Dreamstime.

Monday, November 19, 2007

ETTL Autoexposure

It took me some time to realize what was going on when I first started using my 580EX flash. I would consistently get what seems like underexposure. I almost never shoot without bouncing the flash off something, but even in direct flash result don't seem consistent.

This is a sample shot with no flash exposure compensation. I had the main dial set on manual - 1/80s at f/5.6. No ambient light is coming through, this is all bounce flash. Note the histogram has nothing in the top 1/4, where the most information is stored (in terms of digital 'bits').

At first I thought it was because when the flash is bounced, there isn't enough light, but when I set the flash to manual on full power (1/1), the scene ends up almost totally white. This tells me I have plenty of power, even when bouncing.

Here is a second shot with the exposure compensation for flash turned up to +1 1/3. This completely changes the histogram to look 'right'. Data all the way, and more what you would expect in a scene like this. Note that I'm not claiming this is exposed properly, but nothing is blown out, and you can pull back a bit on exposure after the fact. In the previous case, if I pull the exposure up, I get a lot of noise in the shadows with my 20D.

I find this to be the case with my Canon flash when used in ETTL. I almost always need overexposure on the flash to get a properly exposed shot. I typically run with +2/3 when shooting in auto.
The bottom line is that you need to go manual if you want to nail the exposure every time, as ETTL shifts around a bit depending on the brightness and reflectance of the subject.

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