Moreover, there are CERTAINLY offerings from "substitute" manufacturers that are at least as good as Nikon offerings, and sometimes better. Zeiss is not appropriately called "substitute" - their offerings are world class. You'll read a lot about how lousy Sigma is, but they have really been knocking the ball out of the park recently. Some of their stuff is pretty much universally agreed to be better than offerings from Nikon, Canon, etc. Tamron's SP series lenses are excellent too. As you might guess, though, most of these top quality ones are, well, relatively high priced, in absolute terms, anyway, even if some of them may cost less (perhaps much less) than the Nikkors.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!