Quote Originally Posted by ashmadux View Post
if its good on the rebels, its good everywhere.
Sorry, but Rick is correct - this logic is unsound. My 17-55mm f/2.8 IS was perfect on my T1i/500D, and front-focuses on my 7D. My 100mm f/2.8L Macro IS back focused on my T1i (which was a significant factor in my decision to get a 7D, for AFMA). On the 7D, the 100L is perfect, but on the 5DII it front focuses. Each lens-body combination is somewhat unique. Take a pair of lenses that both front focus on one body, put them on another and one may still front focus while the other back focuses.

Also, an f/4 lens isn't the best 'gold standard'. The deeper DoF at that comparatively narrow aperture will mask AF errors that would be evident with faster lenses.