Have done a quick look of the center to corner sharpness of this lens. The image file is a crop, where the actual center of the taken image is top right, and a corner is at bottom left. Taken at 400mm, f5.6, totally perpendicular to the test target.
I know that this is quite a rough test, but if anyone has any experience with this lens - can you tell me if this variation between the center and corner is usual or not. I'm used to my 400 5.6 prime, which does not show nearly such variation. Thanks for your comments.