So that explains it. I'd been bothered by how my critical focus seemed to be off a bit when shooting wide open. The eyelash instead of the pupil, that kind of critical. I turns out I can adjust my camera body to compensate for autofocus error.
On the Northlight Images site there's a cool tutorial on how to make the adjustment. This only applies to Canon high-end DSLRs (1Ds3 and 5D Mark II), where you can alter the AF Microadjustment under Custom Functions.
From the site you download a target image. Display it on your LCD screen at 100%. Align your camera parallel to the screen (at >50x the focal length). I used my laser enlarger alignment tool, but that borders on the obsessive. Using Live View, you focus on the computer screen. A moire pattern forms when critical focus is achieved, which is caused by the interaction between the computer and the camera LCD pixels (clever, eh?). Then you turn off Live View, and use auto-focus to focus the camera, while watching the focus display on the lens. If the distance marker doesn't budge, then there's no problem. If it does move, then you need to apply a correction.
For my 24-105mm lens, I had to apply an +8 correction to the camera body. For my 70-200mm I only needed a +1.
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