Hmm - interesting to see for me. So, at f8 you get sharp images from
0.64 to infinity with the 15mm.
This is not the case with my Zenitar 16mm on the D700 at f8. You can see
it e.g. in this recent panorama even at lower resolution:
the robot - look outside the window, left behind the robot).
After carefully re-examining the images, i think it is a slight
mis-alignment of my lens' focal plane settings (maybe the adapter).
Thanks for the enlightment!