Entrance and exit pupil

Fig. 11.1(a) shows a lens followed by an aperture (diaphragm, stop) P′Q′ in the image space. How can we aim a ray in the object space so that it will pass through the diaphragm? To answer this question we image the diaphragm backwards through the lens. This image is labeled PQ in the figure. If a ray in the object space intersects the PQ-plane in a point between P and Q, it has to pass through the image that the lens forms of this intersection point, which will be found between P′ and Q′. So the ray will pass through the diaphragm. If, on the other hand, a ray intersects the PQ-plane outside PQ, it will eventually pass through a point outside P′Q′, i.e. it will not pass through the diaphragm. So any ray passing through the actual aperture P′Q′ must pass through its backwards image PQ in the object space. This backwards image is called the entrance pupil.

When the stop is placed in the object space a similar result holds, in reverse. With the arrangement shown in fig. 11.1(b) we cannot produce just any ray in the image space; only those rays that pass through the image P″Q″ of diaphragm P′Q′ can be realized. The effective aperture found by imaging the actual stop in the image space is called the exit pupil.

In most lenses the stop that determines which rays will reach the image space is buried inside the lens, as shown in fig. 11.1(c).