How does AF work?

[booggerg]

I just got my 20D and when it's in 9-point AF mode, how does the algorithm decide which focus point(s) to focus?

[ed2day]

I don't think it's disclosed to the public. Canon proprietary, etc. I've heard that it tends to pick the closest item under an AF point and assume that's the subject, but don't know for sure.

[tim]

I've read it picks the closest point, though I don't know how. Read this (http://www.fredmiranda.com/forum/topic/241524) and there's general info somewhere on http://howstuffworks.com

[Tom W]

I hate to link to a competing forum, but the writeup by RD Kirk is pretty good:

http://www.fredmiranda.com/forum/topic/241524

[ScottE]

In AI-Servo mode it generally starts focusing on the centre sensor and then switches to other points if the subject it is focused on moves off centre.

[ed2day]

I hate to link to a competing forum, but the writeup by RD Kirk is pretty good:

http://www.fredmiranda.com/forum/topic/241524

Thanks, Tom. I was looking for that link and couldn't find it. That's about as good as you're going to get.

[tim]

I hate to link to a competing forum, but the writeup by RD Kirk is pretty good:

http://www.fredmiranda.com/forum/topic/241524

Beat you to it by 2 minutes in my post just about yours :p