While doing some corner sharpness tests with my newly acquired 17-55mm, I notice the uneveness in light falloff on the frame edges. The light falloff is more important on the right side than on the left side. The same uneveness appears at every focal length. It's evident at f/2.8 and tends to diminish with smaller apertures (it almost disappear at f/8) Here's a sample of the phenomenon. 35mm, f/2.8 - NEF converted as is to Jpeg by Nikon View -no sharpening, low contrast. What can be the problem? Thank you.