I just did some autofocus testing on my 17-55 f/2.8 because I was getting a lot of images with the focus just slightly off. I printed off a test pattern and tested the lens at f/2.8 by taking an image focused in live view and comparing it to an image auto-focused normally (using center point).

100% crops of test images are below. I also tested with the same setup using 2 other lenses (70-200 f/2.8 IS II and the 18-55 kit lens) and saw no discernible difference between the images. The autofocus appeared dead on for those lenses, which leads me to believe there is no issue with my camera. So my question, is how should I go about getting this fixed? Just send it to Canon (and if so, any idea of how much this costs)? I bought the lens used off eBay about 2 years ago.

20140826-1447 by sambisu, on Flickr
20140826-1446 by sambisu, on Flickr