The entrance pupil of an optical system is the image of the aperture stop. If there is no physical aperture stop, the entrance pupil is an image of the lens or mirror itself. When it is unclear which element is the aperture stop, the image which subtends the smallest angle at the axial object point is the entrance pupil.
The exit pupil is the image of the aperture stop as seen from the axis on the image plane through interposed optical elements.
Pupils are particularly useful when used in conjunction with the concept of chief ray. A chief ray is any ray from an off-axis object point which passes through the center of the aperture stop. The chief ray enters the optical system along a line directed toward the midpoint of the entrance pupil, and leaves the system along a line passing through the center of the exit pupil. All rays pass through the entrance and exit pupils.
Chief Ray, Entrance Pupil, Exit Pupil
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