55mm f2.8 Micro Nikkor
One valid criticism of the 55mm F3.5 lens had been that all of its formulation corrections were calculated for a 1:10 reproduction ration which was admittedly far away from the bulk of its often intended applications. The F2.8 re-formulation of the lens was a very successful attempt to satisfy this complaint, though many users of the time argued that the original F3.5 was actually a better solution within its immediate intended calculated formulated range of 1:10 reproduction ratio and thereabouts.
Where the 55mm F3.5 delivered a flat field to flat field mapping for a 10 inch subject to a 1 inch image at the film plane; the new F2.8 formulation purported to do the same over its entire focusing range from a 1:2 reproduction ratio to infinity. Below 1:2 there existed some controversy as to whether the recommended PK-3 ring or an extremely well corrected telextender worked best in the 1:2 to 1:1 regime.
BTW, anecdotally, it was the author's own observation at the time that the original 55mm F3.5 was actually sharper at 1:10 than was the newly formulated F2.8 species; but this may be no more than a manifestation of a combination sample variations coupled with the author's prejudice.
Version 1:
AI December 1979, beginning with SN 179041 Nikon introduced the "floating element" 55mm F2.8 AI version which was enough cosmetically similar to the 55mm F3.5 that it was effectively replacing, that the two are easily confused on casual inspection. This version of the lens was very short lived since in following February of 1980 it was effectively replaced
Version 2:
AIS onset serial number 186211 which differed only by having the precisely cammed mechanically compensated F stop communicating lever in the mount surface area along Version 1: by the almost identical AIS version 2,with the characteristic curved groove communicating AIS capability to the camera. This version remained the mainstay until the brief advent of version3.
Version 3, the auto focus 55mm F2.8 which is often identified as optically identical but probably was not since it was "floating element" optically compensated over effectively twice the range of its predecessor. It was enormously dissimilar mechanically and may well have been much more closely related to its 60mm F2.8 successor than its 55mm F2.8 AIS immediate predecessor.
