In a centered
optical system, the aperture stop is the
diaphragm that defines the narrowest conical
beam traveling through the system and having the intersection between the
object plane and the optical axis as its
apex. Together with the
field stop, the aperture stop defines the
throughput of the system. The
image of the aperture stop in the
object domain is the
entrance pupil. The
image of the aperture stop in the
image domain is the
exit pupil.