"This lens is sharp wide open and from corner to corner even on a full frame body. There is little improvement when stopping down - the f/4 IS starts out sharp wide open.
The non-IS 70-200 f/4 is similarly sharp in the center over the mid and long portion of the focal length range, but the f/4 IS is sharper in the center through 85mm or so and is noticeably sharper in the corners over most of the focal length range. These two lenses are most similar at 200mm. These differences are reduced as the subject distance is increased (the non-IS lens performs more similarly to the IS lens). The f/4 IS and f/2.8 IS 70-200mm L lenses are similarly sharp at the same aperture and focal length settings. "