It is frequently asserted that the 400mm L f/5.6 is very sharp and a much better lens than than the 100-400mm L IS. As someone who has used both, I know that they are of very similar sharpness, and they are not as sharp as sometimes stated. So, here is a summary of quantitative measurements of both from three very reliable sources: Photozone, SLRgear and Canon MTFs. In all cases, the 100-400mm is at least as sharp in the centre, which is what counts if you are using them for bird photography. For the fun of it, I have also compared them with the now out-of-production Sigma Apo Tele Macro f/5.6. Whereas, the two Canon lenses hover in the "very good" category of Photozone, the Sigma reaches the excellent - and it really is much better. If Canon had put in some effort, they could have made a super sharp 400mm f/5.6, preferably with IS. I wonder why they never did?