The MFAs datas are stored in the body, that I know never in the lens.
Sorry Fred, maybe you've misunderstood, but there is some focus position data information held in every EF lens.
From what I've read, it's effectively the back focus setting for the lens. Something I deal with on a daily basis with broadcast lenses.
This data isn't changeable by users at all, it's set in the factory or can be adjusted by Canon's technicians in one of their service facilities. Which is why there seems to be so little information about this on the web.
Since the early days of the EOS system it was possible for Canon to match lenses to particular bodies where manufacturing tolerances combined to make the lens/body not focus accurately. This pre-dates any of the recent in-camera MFA options that users can set themselves.
You'd only ever need to know about this if you needed to get a misbehaving system in shape or, as in my case, you're hacking a third party lens with a chip to add EXIF data and the chip has user adjustable focus confirmation data held in it.
My curiosity at the moment is working out which setting is finer, in the lens or the camera body, or if they work in the same units ?
I'll be able to work this out myself when I've fitted the chip, but if that information is already out somewhere it might save a lot of tedious testing here.
Optimising the data for the lens is the most preferable option as I understand that my lens in question doesn't work very well with the default settings in the chip. If I hire it out it would be annoying having to explain the problem to the hirer each time, rather than it just working as expected like the TSEs.