The Canon 300mm f/2.8 is one of the sharpest lenses we've ever tested. This is a lens designed to be used wide open at f/2.8, and it shows in the test results. On a subframe 20D at f/2.8, the far right edge of the frame barely reaches 1.5 blur units, and the rest of the frame edges just above 1 blur unit. This performance improves (as if it had much room to improve) by f/4, where it is essentially tack-sharp across the frame. It's just as sharp through f/5.6-8, and by f/11 we begin to see the effects of diffraction limiting, though at this level to say that the sharpness is degrading needs some qualification. Sharpness hits 1.5 blur units at f/11, and 2 units by f/16. It's possible to shoot at f/32, but here you actually encounter some legitimate softness issues at 5 blur units across the frame. This really isn't a lens you're going to be shooting at f/32, though. Read the whole review here. You can buy the lens here