
Entrance and exit pupil
Entrance pupil
The entrance pupil is the paraxial image of the aperture stop as viewed from the object side of the lens. In other words, the entrance pupil is where a ray starting at the object should be aimed in order to hit the center of the aperture stop, using paraxial propagation. In the figure below, the entrance pupil is after the first surface of the lens, in which case it is a virtual image of the aperture stop. It is also possible for the entrance pupil to be before the first surface of the lens. Note that the entrance pupil is not the same size as the aperture stop in general.

Exit pupil
The exit pupil is the aperture stop viewed from the output side of the lens (again we are talking about paraxial imaging). When combinining two systems, it is important the exit pupil of the first system be similar in size and close in position to the entrance pupil of the second, or there will be loss of light.