Nikkor 50mm F1.4's -----
Ist: old chrome or black front, knurled metal focus ring (vignettes to F4,
poor edges until about F2.8)
2nd: early rubber ring non-A.I. (extremely sharp in center by F4 at expense
of edges, vignette cured)
3rd: late non-A.I. (current version, which is fine by F2 except very far
edges and corners, which are not terrible)
-50mm F1.8 differs in that it is more even in sharpness to corners at F2,
but over all contrast is lower.
-50mm F1.2 differs in that it is better in the center at F2, but worse
on the edges, though at F1.4 it is contrastier than 1.4's. It is also
very good wide open over most of the frame at mid-close distances, which
makes it a very useful interior people lens.
All perform about the same at F5.6 and smaller, with traces of above
characteristics remaining at smaller stops.