I've got both the F50/1.7 and the F50/1.4 and there isn't any difference in the quality of construction. They tend to be pricey here in Australia but that's probably because just about everything sold with a zoom, not a 50mm prime. Both the F and FA 50mm lenses are quite rare secondhand. There are bucket loads of ST, SMCT and M series, while the K and A are less common. The K 55/2 and A 50/1.2 would be the rarest.
Paul Ewins Melbourne, Australia
