Originally posted by slackercruster Depends. I got a 17 with a speck of dark dust in it. Shows up like hell in the sky of each photo. Can't say about knicks though.
Having dust, or scratch inside the lens is a wholly different thing, than a scratch on the front element. The last usually won't show in the final image. The excemption from the rule might be an extreme wide angle which you shoot stopped down, in which case a scratch might leave a visible mark on the image.
Whether dust inside is visible depends on the concrete position in the last. Usually, the nearer it is to the rear lens, the easier it is visible. big dust specks in zooms might get visible, if they are on the rear of the variator - but in fact I have never seen dust specks laving visible traces. And my 300/2.8 also has a scratch on the front element, which bears no consequence at all.
Ben