Flash metering with 10D

[Jesper]

I've just received my 550 EX and ST-E2, and I already had a 420 EX, so now I have a nice and flexible flash setup.

I've noticed that the flash exposure is not always what I expect it to be. I have studied the Flash Photography with Canon EOS Cameras (http://photonotes.org/articles/eos-flash/) but it's still not very clear to me how metering works on the 10D with flash.

Does the 10D always use center weighted metering when using flash?
Is it true that it doesn't matter what metering mode I choose on the camera (i.e. I can't use partial or evaluative metering for the flash exposure)?
Is the flash metering also tied to the selected focus point?

[scottbergerphoto]

Jesper,
The metering of ambient and flash metering is separate. In any but the Basic Zone modes, you can select any metering mode for the ambient light. The flash metering for the 10D is controlled by ETTLI. It is biased to the active AF point unless you switch your lens to manual focus, which forces the camera to average the measurement of the pre flash at all the focusing points. One way of accomplishing this is to use custom function 4 to put the AF on the * button. If you focus with * and then release the * button, there is no longer an active AF point and the flash will based on an average reading.
Scott

[Jesper]

Thanks Scott,

So the metering mode I select only applies to the ambient metering. When I'm not using flash, I usually do the following: I almost always use partial metering. I try to meter for the brightest part that will be in my image, so I point the camera at the brightest part, so that it covers the center area (circle) in the viewfinder and meter with the * button. Depending on the brightness of the part, I dial in more or less + exposure compensation. Then I recompose, focus (by half pressing the shutter button) and shoot. I almost always use the center AF point. This always gives me exactly the exposure I want.

When using flash, the E-TTL measuring pre-flash fires when I press the * button as expected (I'm using FEL).

But when I'm using flash, the camera doesn't use just the partial metering circle to meter the flash exposure off of. Why does it work with such a fixed average / AF point emphasis metering pattern, instead of using the metering mode I choose.... it gives me the feeling that I can't always control the flash exposure very accurately...... :? what's the best way to accurately control Canon E-TTL flash exposure?

[Motorsports Photo]

I've noticed that the flash exposure is not always what I expect it to be.

Me too!

Regular exposure is not what I expect it to be either, so I've had to go to manual. :(

For flash I set the exposure adjustment (usually to "-" ) to get the image I like and then shoot away.

-Pete

[rodbunn]

The best way to use the Flash exposure is to point the middle
of the lense on a grey card in front of the subject and push the
FEL "*" button. Then recompose the "active" button over the
point you want to focus on and push the shutter button half way
to focus. THEN re-compose and fire. I know, this isn't very easy
or possable in REAL LIFE, but that's the BEST way to get the flash
to put the right amount of light (per your choice of apateur) on the
subject.
If you have a hand meter, that's better. Use the flash in manual
mode and set the amount of flash you want to hit the subject. Then
set the camera in manual to what settings you need for the "result"
you want..... Good for portrait work, consistent results.

Otherwise, in real life if you point your camera at a subject, push the
shutter half way to focus, then re-compose and fire; the flash will read
where the "active point" is (when you fire) for it's reading.... So, if the
point is over a black or white part of the frame, it is over or under exposed !
It's not easy to shoot with the flash on these digital cameras,
it takes some thought . . . . .

Good luck,

Rod Bunn
XposeU.com

[scottbergerphoto]

Thanks Scott,

So the metering mode I select only applies to the ambient metering. When I'm not using flash, I usually do the following: I almost always use partial metering. I try to meter for the brightest part that will be in my image, so I point the camera at the brightest part, so that it covers the center area (circle) in the viewfinder and meter with the * button. Depending on the brightness of the part, I dial in more or less + exposure compensation. Then I recompose, focus (by half pressing the shutter button) and shoot. I almost always use the center AF point. This always gives me exactly the exposure I want.

When using flash, the E-TTL measuring pre-flash fires when I press the * button as expected (I'm using FEL).

But when I'm using flash, the camera doesn't use just the partial metering circle to meter the flash exposure off of. Why does it work with such a fixed average / AF point emphasis metering pattern, instead of using the metering mode I choose.... it gives me the feeling that I can't always control the flash exposure very accurately...... :? what's the best way to accurately control Canon E-TTL flash exposure?
If you take pictures that contain very bright (white dress) or very dark
(black tuxedo) it's possible to get widely varying flash exposures depending on which AF point is active and what it's on. The best way to avoid this problem IMHO, is to separate exposure from focus using Cfn. 4, and put AF on the * button. That way once you achieve focus, you release the * button and the flash exposure that occurs when you depress the shutter button is averaged across all the AF points.
There is nothing wrong using a grey card if you are in a situation where that is possible. It's not practical in a spontaneous/fluid situation.
Scott