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The component S1 of the system, situated between the aperture stop and the object O, projects an image of the diaphragm, termed by Abbe the ``entrance pupil''; the ``exit pupil'' is the image formed by the component S2, which is placed behind the aperture stop.

Since the maximum aperture of the pencils issuing from O is the angle u subtended by the entrance pupil at this point, the magnitude of the aberration will be determined by the position and diameter of the entrance pupil.

This distance replaces the angle u in the preceding considerations; and the aperture, i.e. the radius of the entrance pupil, is its maximum value.