Originally posted by JinDesu I actually didn't know that the center one of the 5D Mk2 was better than the k-5. However, I do know that quite a lot of people complain about the 5D's AF performance, and how the new Mk3's AF system is a huge upgrade (if nothing else much was) - that's why I was a little confused as to why they put this in the 6D, and not like the 7D's AF system.
I think people complain because of the other 8 AF points that are just linear f5.6 points while the central one is a crossed f2.8. They are less accurate, less sensitive (don't work in dim light) and also need to "catch" someting quite linear depending on their orientation (ie if the AF point is horizontal you try to have it focus on a vertical line).
Depending on the situations you can't/it's difficult to use the central point and recompose, typically for portraiture 'cause you would back focusing.
Well, now I'm talking about a 85/1.8 @ f1.8. At 3m the DOF is 13cm (5 inches)... The thing is that in good light conditions you can aim the eye. Just have to pay attention to hair, ears, anything that would be far more contrasty at the edges of the focusing zone...
Well, that's what I noticed from both the 5D and 5D2 (same AF).