The 200-400mm is perhaps most likely to be found on high-end full frame camera such as the EOS-1D X and the EOS 5D Mark III. Test results on full frame are little short of extraordinary - the lens comes ridiculously close to matching Canon's very best primes. Click for comparisons with the EF 300mm f/2.8L IS II USM and EF 400mm f/2.8L IS II USM in our lens widget.
Sharpness results are remarkable - the lens delivers superb results right across the frame wide open at F4, all across the zoom range. There's essentially no need to stop down for sharpness - there's little difference from F4 through to F11.
Chromatic aberration is very low. If you look really hard you may see a little fringing at each end of the zoom, but is simply won't be a problem in normal use.
Vignetting is on the whole pretty low. It may possibly be visible in some situations towards the long end of the zoom.
Distortion is essentially nonexistent at any focal length.