From left to right: AI (later version), AI-S, AI-S pancake (60cm), AF (2nd version), AF-S.
Note that these are by no means all versions of the 1.8/50. There was a earlier AI version, a version of the pancake focusing to 45cm, an earlier AF version with a distance window, and the AF-D. In addition to that, there was the Nikon Series E, with two versions.
Except for the AF-S, all these lenses (even the Series E) have the same optics - 6 elements in 5 groups. The coatings have changed over time, though.
Copyright © 2013 Christoph Breitkopf - last edit: 2013-02-10