Re: AF w/70-200 2.8 is
[ QUOTE ]
Give the AF sensor some more contrast to work with on your next test.
[/ QUOTE ]
Michael makes a very good point here. The chain has much more contrast than the focus point. I've often noticed this happening when shooting blond people in backlit situations. Focus point right over the eye but sometimes the AF focusses on the rimlit ear about 2" back..... Not enough contrast in the eyebrow/lash department maybe.
Happens with all my telephotos now and then and I'm wondering if its a Canon AF foible (I remember a 24-70 thread about the same sort of problem affecting that lens in particular).
I used to have CFn17 on my 1DMKII set to "1" (expand to 7AF points) but wondered if this was the problem, i.e. AF cant get a lock so shifts outwards and finds a high-contrast edge to lock to. Have since set Cfn17 to "0" (single AF point) and it seems a tad better though I now routinely "touch up" the AF (when theres time) with the MF ring.
Of course, I got paranoid about this happening every now and then and so ran the old 45 degree lined paper test. Under controlled tripod conditions in good light the AF is spot on as is the MF can't get it to fail. That's why these tests are a bit suspect IMHO, they're not "real world" shooting where you have 5 mins to set the lighting and get out of there!
I think the motto must be "know when to trust your AF and when not to" [img]/ubbthreads/images/graemlins/smile.gif[/img]