
STOPS, PUPILS, AND WINDOWS 93
Virtual
exit
window
Image Aperture stop
Figure 4.8 Exit window. The exit window limits the field of view as seen from a point in the
exit pupil.
In order to determine the light-gathering ability and eld of view of even this simple system
without this level of abstraction, we would have needed to consider multiple rays from differ-
ent parts of the object in different directions, trace each ray through the lens, and determine
which ones were obstructed along the way. Furthermore, if the result is not to our liking, we
have little intuition about how to modify the system. Even for a system consisting of only a
lens, an aperture, ...