AF w/70-200 2.8 is

[Michael_Sutherland]

Phil,

Give the AF sensor some more contrast to work with on your next test.

[JohnBoyes]

[ QUOTE ]
Phil,

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]


J

[Will_Perlis]

' I think the motto must be "know when to trust your AF and when not to" '

Right. And one thing you can't trust is the size of the red rectangle in the viewfinder. For sure, the center AF sensor is considerably larger than its red rectangle. It's easy to verify this if you have a featureless white wall with one black dot or line on it handy. IMX it's always possible to get focus lock before the contrasty object is within the red.

(And yes, on this 1DMk2 the center red is centered in the viewfinder and right in the middle of the grid screen's cross-hairs. My 1v does the same thing but it's a bit off center so the locking isn't qute symmetrical around the red perimeter.)

[Phil_Adair]

Hey guys

Thanks for the suggestions. Getting to know the AF system is very important to me. I've done some one shot mode testing and the results are spot on as I would expect. Looks like the AF system overall is an area that really differs from Brand N.

It also looks like C.fn. 21 is related to the fire on focus vs release option on Brand N.

Seems like I will be spending some time on C.fn's 4, 19, and 21

[Christoph_Jung]

have you checked http://www.canon.photoworkshop.com/canon/index.html ? Click on "Getting the most..." on the bottom and then take the first document. You find a very good introduction on Canon AF, the related C.Fn. and P.Fn and - what I like best - not only how things work, but also why.
Christoph

[Phil_Adair]

There was a loose nut between the viewfinder aand shutter release

New test

Thanks for the pointers it's much better now. [img]/ubbthreads/images/graemlins/laugh.gif[/img]