05-12-2015, 11:20 PM
It probably has to do with the optics around the OS/VR unit not being "properly" desensitized to tilts. Theory is that the IS/VR will only translate the group in the X-Y plane with no Z movement or tilt, but in practice this is never the case so the optics there must be insensitive to tilts. If this is not done with sufficient care, the OS/VR will impact image quality in a way that is time consuming to predict with computers and impossible to predict by eye.