Flash and focus points

[kirios]

The article at http://photonotes.org/articles/eos-flash/ says both TTL and E-TTL exposure are linked to the current AF focus point and off-centre focus points should be selected instead of using the centre point and recomposing.

Would this be equally true for the 400D as for a camera like the 40D which has multiple cross-type sensors?

What is/are the disadvantage(s) of flash photography using the centre point and recomposing?

I'm reading through the links at http://photography-on-the.net/forum/showthread.php?t=138907 at the moment but haven't come across this yet.

[René Damkot]

The 400D and 40D use ETTL-2 which is not AF point based, but subject based.

[jrsforums]

The 400D and 40D use ETTL-2 which is not AF point based, but subject based.

Hi, Rene....

I know what you mean, but not very one might know that the "subject" in "subject based" is determined, in "evaluative flash mode" by the reflected light of the pre-flash. In "averaged flash mode" ETTL-2 is not "subject based", but meters the entire scene.

This is described here: http://photography-on-the.net/forum/showthread.php?t=46311&highlight=ettlii

(BTW....for the 400D and 40D there are 35 meter points)

[Curtis N]

Rene is correct. E-TTL II doesn't bias the exposure on the active AF point.

The photonotes.org article kirios mentioned is a very good resource, but it was written before the advent of E-TTL II. It has been updated somewhat, but apparently there are parts that still need clarification.

[jrsforums]

Rene is correct. E-TTL II doesn't bias the exposure on the active AF point.

The article you mentioned is a very good resource, but it was written before the advent of E-TTL II. It has been updated somewhat, but apparently there are parts that still need clarification.

Curtis...

I assume you a referencing the 'Photonotes' article.

The article/thread I referenced is specific to ETTL-II and supports what Rene said...and didn't say.

[Curtis N]

I assume you a referencing the 'Photonotes' article.Correct.
Previous post edited for clarity.
Sorry for the confusion.

[kirios]

Thanks for the clarification. The Photonotes articles wasn't very clear on this point. I was also confused by the 420EX review at http://emedia.leeward.hawaii.edu/frary/elan7e4.htm which must have been written before ETTL-II was introduced.

I was curious about this because not all the focus points on the 400D have cross-type sensors (unlike the 40D) and I assumed that an off-centre focus point would be less reliable for AF in low-light. From the article/thread linked by jrsforums, it appears that 'focus-and-recompose' shouldn't be a problem on a 400D/420EX combination.

[Curtis N]

From the article/thread linked by jrsforums, it appears that 'focus-and-recompose' shouldn't be a problem on a 400D/420EX combination.I agree that this technique should not cause any particular problems with flash metering.