What you're adjusting for are the combined manufacturing tolerances of the body and the lens.
Change the lens and you'll be changing the net tolerance, i.e. each lens/body combo is unique, so change one, even for another otherwise identical model and you've created a new unique combination. AF Adjust allows you to fine tune that combination negating (hopefully) the manufacturing tolerances of both bits of equipment.
Note, though, that sometimes this won't be possible, and often leads to lenses being described as 'bad copies'. Often, however, that same 'bad copy' lens will work fine with another body simply because the sum of the tolerances is within the range of the AF Adjust.
The bodies and lenses are built to an engineering specification as well as price. The closer you build to perfection the costlier it becomes, so all manufacturers have design tolerances within a range which are acceptable for the price they charge for the item and the customer is prepared to pay.
PS. Not all 'bad copy' lenses are down to tolerance issues; sometimes they're damaged, really badly put together, etc, and are truly 'bad'.
Last edited by JohnX; 05-16-2013 at 03:00 PM.
Originally posted by BethC 
Similar Threads 
