Why am I always reading that people have a 'very sharp copy' of a lens, especially when its something like a 24-105 f4 L? In Bryan's review of this lens he says:


... the Canon EF 24-105mm f/4 L IS USM Lens is very sharp wide open and shows only very slight improvement when stopped down.


So what does this mean then? Is there really that much play in the manufacture of the L lenses such that the one reviewed was very sharp but others might not be? Also, how do these people know they have a sharp copy vs that lens just being sharp, period. I see people making this comment *way* too often for the answer to be that they've owned multiple copies of that lens.


Or am I missing something?