kenyc wrote in post #5859850
True, but the point is that if you have a lens that is BF or FF then it needs adjusting, it may not work correctly on your other bodies.
That's certainly true.
But my point, actually, is that with today's technology, the kind of lens adjustment that typically needs to be made in order to fix a backfocusing or frontfocusing problem is entirely unnecessary and can easily be overcome with a bit of programming. The necessity for such adjustments should be limited to compatibility with older bodies only.
I'd even go so far as to argue that even older bodies, such as the 20D, should have enough horsepower to handle a firmware update that takes care of this issue properly, but that assumes that the change I have in mind can be accomplished with firmware only. It depends entirely on how much of the autofocus system is run from firmware and how much is hardcoded in the circuitry of the autofocus system.
A good description of how the autofocus system works is here: http://www.fredmiranda.com/forum/topic2/241524. In it, they say:
When you half-press the shutter release (or the * button, if you've used the custom function to move focusing control there), the activated AF sensor "looks" at the image projected by the lens from two different directions (each line of pixels in the array looks from the opposite direction of the other) and identifies the phase difference of the light from each direction. In one "look," it calculates the distance and direction the lens must be moved to cancel the phase differences. It then commands the lens to move the appropriate distance and direction and stops. It does not "hunt" for a best focus, nor does it take a second look after the lens has moved (it is an "open loop" system).
Closing the loop should be straightforward: perform a second check of the autofocus sensor. The image is in focus if it detects no phase difference after the initial focus operation, but if a difference is detected, simply command the lens to move again by the amount now dictated by the current autofocus sensor reading. Repeat until focus has been achieved.
Maybe we were just more naive. 
