Not many will be interested on this post. I made a comment that the 15mm 3.5 ais, my copy, is hard to focus for top performance. Now that I own many other 15mm to 20mm wide angles, it is evident to me that other lenses just need to be focused at infinity and closed down and the frame becomes sharp within the lens curvature of field.
The 15mm 3.5, my copy being like new and tested by Nikon El Segundo, needs to be accurateliy focused closer than infinity, not too far, not too close, and it will give a sharp frame at f/8 and f/11. At bigger apertures it works at closer distances but not at infinity. My other wides work at all apertures. Ken Rockwell says something similar.
In any case, if you put the time in and shoot at a couple of different focus distances you can get this good with the lens:
