Well at infinity in the mid-ranges it's not extreme corners and not even close to ok. If one wants to save the corners with this one it seems you'll have to focus a bit past center's infinity and sacrifice the middle some. Here is a 50mm example at F4.
100% crops. Number 1 is from the center and 2 with it reframed to the upper left corner....so that closer tree is actually even further out in the frame. Yet once it is in the corner the closer tree gets better as the further stuff goes to hell. The field curvature is just bending back towards the camera a good bit. Does it both sides like that and from other reading it sounds like just design related. http://www.extremeinstability.com/1.jpghttp://www.extremeinstability.com/2.jpg
But again, on number 2 that is the furthest upper left corner. The tree is pretty dang great for extreme corner, but alas infinity areas have gone to hell out there. If you focus using the corner out there you can pull them in good but you then sacrifice the middle sharpness.
Starting to think this lens isn't out of whack and that is just how it is using it at infinity.
More talk on it here: http://www.dpreview.com/forums/post/52164436
THink that was the place I saw someone setting their afma purposely off a hair to help the corners.