Canon 450D autofocus problems

Marc dbr wrote:

I shoot a lot of outdoor portraits with my Canon 450D (Rebel XSi) and Canon 70-200 f4L IS USM and the results are very nice when the focus is right on.

But only 20 to 40% of my shots are tack sharp, most of the time when my subject is not moving. The action shots and kids portraits are generally oof by a little bit.

I am convinced that my techique is good because I never shoot with the 9 focus points on auto, I'm always selecting one of the spots.

With moving subjects do you have the AF mode set to AI Servo? Are you allowing it to track your intended subject by holding at half-press for a moment or two before full press to capture? And/or do you have the Drive set to Continuous and capture a burst to give the camera time to "catch up" to your moving subject since it continues to track between shots of a burst?

I've also read that you could calibrate your lense(s) but it is not available on the 450D. I'm also planning to uprgade to a 5DIII when the 5DIV comes out, but until then (october I guess?) what do you recommend me to do to improve my AF accuracy?

The 450D does not have MicroFocusAdjustment, but you can have your lenses and/or body calibrated by Canon Service (or other shop) if that's the problem.  But since you get good AF with still subjects I doubt it's lens calibration causing the issue with moving subjects.

Thanks in advance.