If you go to the the tools and review the two side by side. The F4 will be sharper than the F2.8 in the corners if the comparison is at F2.8.


The 70-200mm F2.8L II is not showing to be just tack sharp at F2.8. Now if the comparison is both at F4 then the 70-200mm F2.8L II is the clear winner.


I would suggest the OP compare the charts at the settings he has used and see if the results seem similar.


http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=404&Camera=453&Sample=0&am p;FLI=2&API=0&LensComp=687&CameraComp= 453&SampleComp=0&FLIComp=1&APIComp=0