The coldsiberia.org tests yeild some interesting results, as the lens does not appear to be that sharp at f4. I know that the f2.8 is very nicely sharp even at 2.8. Last fall someone posted results from three different canon f4 17-40L lenses and one was significantly sharper than the others. I wonder if this could be the case in this test, could it just be a slightly "bad" L lens, or would all of the 300 f4s behave like this? If there are good and bad L lenses in the same models how hard would it be to get Canon to warranty it? Buying a professional lens at a professional price shouldn't have to be a gamble.