wife bought AF-S Nikkor 14-24mm f 2.8 G ED. This is 2nd new one after 1st trying a 9++ used. Neither of us is "technically" inclined but have tried all three on two different Nikon bodies (D7100 & D610). Same result: a distinct shadow in the low center of the images. The first two units were returned but we're holding the 3rd hoping that we, not Nikon, are the problem. Regardless the mode, flash or not, same result. Any help???