The Canon EF 400mm f2.8L IS II USM is one of the sharpest Canon lenses scoring 21 P-Mpix when measured on a Canon EOS 5D Mk III. This negligible difference between the sensor Mpix and the perceived sharpness is excellent. All of the measurements are good: the transmission is very slightly slower than stated; there is one tenth of one percent distortion! Too little to see and vignetting at 1.1EV is manageable, easily fixed with DxO Optics Pro. This is a top quality lens.