I have had this lens for a few months but never made a reasonably accurate test, so I decided to determine if that field curvature issue that is often attributed to it is a real problem or can be tolerated.
Results seem to indicate that there is no appreciable field curvature, but that extreme corners are just barely outside of the image circle till 5,6.
Center and most of image appear to be very sharp from 2,8, with only marginal
improvement if any stopping down.
This is the full image.
_DSC2581small by sergio lovisolo, on Flickr
Conversion with PS and ACR not ideal for Nikon (but that's what I have)
Unfortunately, Flickr takes the unrequested decision of over sharpening the images