I did some reading regarding this puzzle of pupils.
In Warren J. Smith, Modern Optical Engineering, 3rd edition, 1990, Chapter 6, we can find the following definition:
In every optical system, there are apertures (or stops) which limit the passage of energy through the system. These apertures are the clear diameters of the lenses and diaphragms in the system. One of these apertures will determine the diameter of the cone of energy which the system will accept from an axial point on the object. This is termed the aperture stop, and its size determines the illumination (irradiance) at the image.
He then proceeds to examples where the aperture stop is given by one lens diameter for one point in object space and the diameter of a different lens for another point in object space. Meaning that indeed what is called aperture stop is whatever constrains your view through the lens when looking at it from a given point in object space. Interesting.
Thus, the aperture stop depends among other things on the object distance. Usually, the aperture stop in a photographic lens is the diaphragm for all object distances on-axis. Off-axis, it is given by the aperture stop or the image of certain edges of glass element (yielding the cat eye-shaped aperture mentioned earlier). In the repro-Nikkor 85/1 however, it might be that ion-axis the aperture changes from the front lens to the diaphragm. What do you think? I'd love to see this phenomenon in person.
