Don't see why anyone is surprised to learn that there is film curvature with
35mm negs in the Nikon scanner.
Every pro lab knows that you have to use glass holders for film when working
to critical sharpness, 35mm or 5x4". and 4000 dpi needs critical
sharpness...
Nikon makes and sells a glass holder for the scanner (and the 8000 too) so
what's the problem?
critical work - with traditional enlargers or with high end scanners,
demands some care and attention, and it's actually slightly reassuring that
the focus is so precise IMO.
It seem this is not really a fair criticism of the scanner. It is intended
for semi-professional work after all.
Use the glass holder, (with anti newton glass if you have that problem) and
you will see astounding differences in your image sharpness.
pg