Recently while photographing the night sky my shots came back extremely blurred on the right side with darkening on all corners. I contacted Nikon support and here was their response:
"Hi Mike, Thank you for submitting your photos. Upon evaluation we have notice that the focus was set to Manual mode. You need to manually adjust the focus on the lens by rotating the focus ring. It's hard to manually determine a focus at night because you cant really see how sharp is the image when looking thru the viewfinder. I would advise you to set the Focus mode to automatic. Regards."
Thanks Nikon, for the manual focus lesson (even though they're telling me not to use it...). They're suggesting I auto focus on tiny points of light with a wide-angle lens in the dark - not possible. How does using manual focus render stars sharp in the center and left side of the image, but totally blurred on the right when all stars are at infinity? They're suggesting auto focus will remedy this problem. And nothing at all about darkening of corners. In any case, perhaps someone here has a better explanation - I suspect a lens element is out of alignment. If not, truly poor results from a $1600 lens.
Click on the image for original version. No polarizer or lens hood was used, which could normally explain the dark corners.
Thanks
Mike