In order to realize a programmed AE, it is innevitable to control the aperture stop of the lens from the camera body and it is the problem how to realize that with many Nikkor lenses accumulated since the old age of the Nikon F.
With the combination of Nikon F2 Photomic S + EE Control Unit DS-11 (photo left) and Nikon F2 Photomic AS + EE Control Unit DS-12, the method of rotating the aperture ring of a lens direct with a servomotor was adopted.
However, this seemed difficult to equip with the FG since it would make the mechanism too elaborate.
After all, almost only method left to control the aperture stop from the body was to move the automatic aperture control coupling lever which was equipped with every lens.
This automatic stop down coupling lever, however, only transmits those two situations to a lens,
1.) when pushed up to the limit position, it fully opens the aperture stop, and,
2.) when pushed down by the power of a spring, it stops down to the aperture value fixed by the aperture ring, but isn't considered about the situation that the movement of the lever is stopped on its way of the stroke.
Therefore, even when this lever is stopped moving at the same position on the way of its stroke, it differs at every lens, to what extent the aperture stop blades are stopped down.
In many cases, at about the position two(2) steps stopped down, the stroke of the coupling lever reaches about the half way, and in the stopped down region (e.g. f/16 or f/22) the movement of the lever becomes very small.
Such being the case, it is difficult to control the aperture by using the stop down coupling lever. Particularly, in the stopped down region, as a very small movement of the lever should be controlled, it would be difficult to insure accuracy.
So Nippon Kogaku started to standardize the movement of coupling lever to prepare beforehand for the days of multi-mode AE to come.
"AI-S Nikkor" lens was it (This naming is used only in Japan (Nippon). By the way, in Japan, among AI Nikkor lenses, "AI-S" type lenses are distinguished from others in that the smallest stop (i.e. the largest numeral) both in the aperture ring and the view finder indication is colored orange).