Should I use the rear nodal point of the last lens instead of the entrance pupil as the center of projection if I want to stitch images at different focal lengths?

I often read that the entrance pupil is the correct center of projection when taking panoramic photos.I am not rotating the camera, but the center of projection is IMHO still relevant in my case:Place Object A at distance x, focus on it, take a picture. Object A ends up at position A' on the image.Shoot a ray from the center of the aperture stop (i.e. assumed center of projection) through Object A. Pick a point on that ray behind Object A and place a scaled-up copy Object B there. Focus on it, take a picture. Object B ends up at B' on the image.In this scenario A' !== B' because of the differences in focal lengths. What is the ray along which I can place Bs so that A' == B', i.e. they create the same image?I am specifically interested in a scenario that replicates the human eye, i.e. Object->Fixed Lens->Aperture Stop->Variable Lens->ImageIn this scenario, changing the focal length of the system only involves the lens after the aperture stop, i.e. changing it does not change the entrance pupil. As a result the ray along which to place Bs doesn't change.In this scenario, is there a better point than the aperture stop through which to shoot a ray at Object A, that minimizes the differences between the images A' and B'? If so, how do I calculate that point?I tried to simulate it with a ray simulator (see images), it is not very accurate so if anybody knows open-source or free software to simulate my scenario I'd be very thankful.Picture 1 Picture 2Legend for the pictures:Object B is behind A when shooting through aperture stop.Object C is behind A when shooting through lens center.A',B',C' are images of A,B,C when focussed on Object A.A*,B*,C* are images of A,B,C when focussed on Object B/C.It looks like C* (via lens center) is much closer to A' than B* (via aperture stop). So is the entrance pupil not the "center of projection" if you need to refocus?Can somebody "shed a light" on this? ;-) via /r/photography https://ift.tt/2W6qH9I