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exit pupil

= image of the physical iris of a lens, as seen through the (image side) lens elements.
https://upload.wikimedia.org/wikipedia/commons/thumb/d/db/Camera_lens_exit_pupil.jpg/1024px-Camera_lens_exit_pupil.jpg
(C) Wikipedia

exit pupil

Here we also see the Chief Ray Angle = CRA between the chief ray and the optical axis. Some sensors like for example from E2V accept only angles between 5 and 20 degrees, say != zero.

If (image side) CRA==0, we call the lens “image side telecentric”.