Certainly the lens may be really good - if you get caught a good specimen. For years, the extreme manufacturing tolerance bugging me all manufacturing, and Canon is no exception. I had a 300, which was worse (also dimmed) when my 4 / 70-200 IS USM with 1.4x extender. So I can only recommend a reputable dealer (eg Amazon ...) to buy, which makes no Heckmeck on redemptions. An objective should be tested immediately after purchase with critical subjects and at all apertures, the images recorded on unsharpened and 100% view (Photoshop: actual pixels) are viewed on the screen, especially in the corners. And then it says: What is my quality claim? I think, in such an expensive lenses like this one you can expect more!
