A few introductory notes:

1) The exit pupil is a real image of the entrance pupil (nominally the objective) formed by the eyepiece, and located in space behind the eyepiece at the eye point distance. This is where your iris must be located in order to enjoy a full and evenly illuminated field.

2) The eypeice is an optical coupler which places the reduced objective onto the plane of the iris. But this is a two-way street; the eyepiece also projects anything in the plane of the exit pupil onto the plane of the entrance pupil (objective.)

3) As long as the iris is as large or larger than the exit pupil, the full aperture of the objective is utilized.

4) When the iris becomes smaller than the exit pupil, an aperture stop is introduced into the system. The eyepiece projects the iris aperture

*onto*the objective, which is

*exactly*like placing a physical aperture mask of the same size there. If the iris is half the diameter of the exit pupil, its image on the objective is half the diameter of the objective.

Due to the location of the projected aperture stop

*in the plane of the objective*, all light from all parts of the field must necessarily pass through this

*one and only*reduced aperture; the area of the objective outside this contributes absolutely nothing to image formation.

5) When the too-small iris moves about in the larger exit pupil, its image also moves about on the objective. If the iris meets the edge of the exit pupil, its projected image meets the edge of the objective, on the opposite side. If the iris half overlaps the exit pupil (50% clipped), its image is half inside-half outside the opposite objective edge.

As before, no matter where the reduced aperture is projected onto the objective, the area outside the projected iris does not contribute in any way to image formation, anywhere in the field of view.

One last remark, about the bottom panel in the illustration which deals with the too small iris/too-large exit pupil. The shaded region bounded by the red rays lying between objective and exit pupil is the full envelope of light fielded by the eyepiece as it forms an image of one point on the edge of the iris onto its corresponding point onto the objective. And conversely, a point on the objective projected by the eyepiece onto its corresponding point on the exit pupil/iris.