Some lenses have been in Canon's arsenal for many years. I'm looking at purchasing a 200mm f2.8, which was first made in 1991 and mechanically upgraded in 1996. So my question is: while Canon has not changed the optical formula of its old lenses, have manufacturing tolerances and other improvements (maybe better coatings) meant that newer copies are better than older copies? In other words, should I buy new, rather than a 15 year old lens in excellent condition?