[Brightcolours]

You have been claiming this: with a front/back focussing lens the focus screen has to be shimmed to calibrate the focus screen to that particular lens.

 

That is what you have been claiming, and that is just weird nonsense. If the focus screen is in place, it is fine for every lens you put on the camera.

 

About AF: you show a remarkable limited amount of understanding of how things work. Evidently the AF sensor needs to be in position. Evidently, the mirror needs to not be misaligned. If either of those are not right, the camera has to be repaired as AF will not be ok for any lens. Most cameras sold are totally fine concerning the mirror alignment and the AF sensor placement/calibration.

 

AF issues with certain lenses then come from either of two things: the lens does wrong sized steps at the end of the AF algorithm, which the camera does not expect and does not check (this is for speed reasons). Or the lens has colours not all focussing on the same plane, and the AF sensor catches onto one of those different focussed colours.

 

In the former case one has to send in the lens for calibration.