Page 193 - Modern Optical Engineering The Design of Optical Systems

176 Chapter Nine

Figure 9.1 The elements of a
simple box camera illustrate the
functions of elementary aperture
and field stops (the diaphragm
and mask, respectively).

of the object. This image is then reimaged at the first focal point of the
eyelens by the erector lens, so that the eyelens forms the final image of
the object at infinity, where it can comfortably be viewed by the eye.

9.2 The Aperture Stop and Pupils
By following the path of the axial rays (designated by solid lines) in
Fig. 9.2, it can be seen that diaphragm #1 is the aperture of the system
which limits the size of the axial cone of energy from the object. All of
the other elements of the system are large enough to accept a bigger
cone. Thus, diaphragm #1 is the aperture stop of the system.
The oblique ray through the center of the aperture stop is called the
principal, or chief, ray, and is shown in the figure as a dashed line. The
entrance and exit pupils of the system are the images of the aperture stop
in object and image space, respectively. That is, the entrance pupil is
the image of the aperture stop as it would be seen if viewed from the
axial point on the object; the exit pupil is the aperture stop image as it
would be seen if viewed from the final image plane (in this case, at an
infinite distance). In the system of Fig. 9.2, the entrance pupil lies near
the objective lens and the exit pupil lies to the right of the eyelens.

Figure 9.2 Schematic sketch of an optical system to illustrate the relationships between
pupils, stops, and fields.