[Brightcolours]

Lenses are NOT shimmed for accurate focus. Lenses may be shimmed to get the focal length correct and the infinity point at infinity, but with AF it has... nothing to do at all. A lens which needs adjustments concerning AF accuracy on a certain camera (often referred to lens calibration) will not get "shimmed"

 

Case in point: a lens with an extension tube can focus just fine.

 

About the 6D vvs 5D mkII: Not only does the 6D have a much better noise profile, without the 5D mkII trademark noise banding, it ialso has a more accurate AF system with more AF settings, WiFi remote live view.