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EOS 40D - Evaluative metering in AI Servo mode

Doug Kerr

Doug Kerr

Well-known member
During my 300D days I was very current in these matters, but the details for the newer cameras (and for the 300D, actually!) have gotten away from me (and I'm not sure I ever knew the answer to this particular issue, anyway).

If we have an EOS 40D set for "all" AF points (automatic AF point selection), AI Servo AF, and Evaluative metering, what AF points does the Evaluative metering system consider "active" to apply its "emphasis " (whatever that is anyway)?

I could imagine possible answers could be:

1. The center one.
2. Which ever one was controlling focus at the time of full-press (as a result of "lateral" tracking.
3. All the ones that would qualify as "active" if One Shot AF were in effect (that is, essentially all those considered to embrace the "subject", based largely on consistency of observed distance at those points in the scene.

Thanks.
 
Doug Kerr

Doug Kerr

Well-known member
We now have a definitive and thorough discussion of this and many surrounding issues from Chuck Westfall of Canon USA:

http://www.prophotohome.com/forum/c...valuative-metering-post469325.html#post469325

I was startled to learn that, even if several AF points "light up", this merely which points on the scene lie within the depth of field surrounding the one point that is used as the basis of AF , and it is only the area around that "principal" point (and we are not shown which one that is) that is given "emphasis" (whatever that is) by the Evaluative metering system.

I am quite certain that in my 300D days, this was not the impression I was given by Canon's explanations of the Evaluative metering system (although of course the details were not completely clear). Perhaps the doctrine has changed. My impression was that the areas around all the "lit" AF points were given "emphasis" by the Evaluative system.

I also think I see in Chuck's remarks (as well as by the language in the 40D manual) that Canon today has a more modest view of when Evaluative metering is most appropriate than at an earlier time (when it seemed to be the answer to everybody's prayer for exposure metering).

Best regards,

Doug
 
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