Originally Posted by Matjazz 
What is centering defect?
This is an example of how bad it can get:
Testing the Sigma 17-70 - Pentax Lenses - Digital SLR Cameras - Steves-digicams.com Forums
A lens has multiple lens elements inside...if any of them aren't aligned exactly parallel, you'll get a "centering defect". What's worse is that you can have this at specific focal lengths and distances as was the case w/ my Sigma 17-70...