Okay I know one here hasn't had much luck with Sigma's focusing dead on, but my trusty 70-200EX works flawlessly on the 20D but wanted to use it on my MrkIIN at the ski hill tomorrow but it's front focusing big time so guess it'll be the 20D tomorrow. Manual focus it's fine.

Anybody know how these things are calibrated? There's a place here in Toronto I can drop it off with but was curious on how they do this.

In the case of my Sigma 120-300 f/2.8, they replaced the main flexboard.

Interestingly enough, many 20D owners reported zero focusing issues with the 120-300, while most 1D/1D MkII(n) owners had intermittent front-focusing...

Here's a simple test, focused on the + and the two lines are about an inch and a half apart and the paper was at about 35-40 degree angle, cropped image:

<img src="http://farm3.static.flickr.com/2386/2285154319_b9a975ea70.jpg">

Even though they had previously had in their hands my 30d.. The tech found that it back focused considerably on a 40d they had..

FWIW, my problem with my Sigma wasn't consistant front/back focus, it was consistant NON focus..

Also FWIW, my 10-20 and 70-200 are FLAWLESS on my 30d.. =D