Based on Bryan's ISO 12233 charts and other reviews, the 70-200mm f/2.8L IS II should be sharper than the 70-200mm f/4L with both wide open, especially away from the center of the frame. I haven't used a 70-200mm f/4L, but I can tell you that my70-200II is very sharp at f/2.8. Stopped down to f/4, it gets a little bit sharper, as you'd expect.


Here's an example from mine at f/2.8, original and a 100% crop:



[url="http://www.flickr.com/photos/dr_brain/5117397391/lightbox/][/url] [img]/cfs-file.ashx/__key/CommunityServer-Components-UserFiles/00-00-00-35-15/EyeCrop.jpg[/img]
EOS 5D Mark II, EF 70-200mm f/2.8<span style="color: #ff0000;"]L IS II USM @ 70mm, 1/640 s, f/2.8, ISO 100


How are you testing? If you're using autofocus, it's quite possible that you need to do a microadjustment. Try testing sharpness using manual focus with 10x Live View. If your copy is not sharp when manually focusing with 10x Live View, I'd suggest exchanging it for another copy.