Figure 1.7 Simple lens system and aperture stop.
In the case of a lens with an internal aperture stop, as in Fig. 1.7, the concepts of entrance pupil and exit pupil are particularly important. If we view the lens in Fig. 1.7 from the object space, we see an image of the aperture stop, known as the entrance pupil. Its position is found by the construction shown in Fig. 1.8. In the image space, the image of the aperture stop is known as the exit pupil, and its position is found using a similar construction.
Figure 1.8 Construction to find the entrance pupil position.