If you look at Bryan's [url="http://www.The-Digital-Picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=687&Camera=453&Sample=0&am p;FLI=3&API=0&LensComp=687&CameraComp= 453&SampleComp=0&FLIComp=3&APIComp=3]ISO 12233 crops for 200mm f/2.8 vs. f/5.6[/url], you can see that at mid-frame and in the corner, the lens is a little bit sharper at f/5.6 than wide open.


[quote=neuroanatomist]I'll try to shoot something with my 70-200 II on my 7D for comparison[/quote]


Here are 100% crops from the extreme corner of a pair of shots taken with my 7D andEF 70-200mm f/2.8<span style="color: red;"]LIS II. Camera-to-subject distance was about 10 feet -DoF at 200mm, f/2.8 is less than 2 inches,so I chose a flat subject (the back of Michael Freeman's Perfect Exposure). Shots were tripod-mounted and lit with an off-camera Speedlite 430EX II placed at a 45&deg; angle about 3 feet from the book and wirelessly triggered with the 7D's built-in flash.


[img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.35.15/70_2D00_200II-corners.jpg[/img]


As you can see, the f/2.8 image is a little less sharp and has a little less contrast. As Jon stated and Bryan's ISO 12233 crops show, a little corner softness wide open is normal for this lens (almost all lenses, in fact). The MTF/resolution tests at photozone.de also show slightly lower resolution at f/2.8 (not just in the corners) with this lens on a 50D.


I should also point out that for the above test I disabled peripheral illumination correction in DPP, and had to bump up the exposure on the f/2.8 image by half a stop to compensate for the small amount of optical vignetting this lens has wide open (even on a crop body). I compared with vs. without PIC, and the correction actually seems to soften the corners by a tiny (very tiny) amount and reduce the contrast as it increases the brightness to compensate for vignetting.


Obviously, since we're all shooting different scenes it's tough to tell for sure, but it looks (to me) like your images have a greater differential between f/2.8 and f/5.6 than mine (or Jon's before they disappeared).