"The service manual from Canon tells how to do it using special
charts and software which I do not have."

Usually that means the backfocus needs adjustment. Yes a very nice pattern is simply wonderful to use, but anytihng with alot of detail will work.

This is a MECAHANICAL adjustment. On a CCD or MOS sensor cam it will be on the lens somewhere towards the back. The older image tube cameras had a clamp you could loosen and move the tube to and fro. The connections all went with it, it was not like dismounting the thing to replace it, which is another whole ball of wax.

Generally, you need to get REALLY sharp focus long, and then bring it in to closeup, zoom in that is. Then without touching the lens focus you adjust it mechanically so it is in focus. Then you zoom out and it will be off. then you refocus it and then zoom back in and set it again.

Lather, rinse and repeat.

But then, you might have a different type of autofocus. there are a few IR types and a couple of ultrasound types. the backfocus thing applies to all of them but newer units may have eliminated the needf ro a backfocus adjust with better menufacturing techniques.

Also, if the unit is ld, some of the older autofocus systems did not really work all that well.