Pupil Position


The size and position of the pupils is closely tied to the position of the principal planes and the aperture. We have already shown pictures of the entrance and exit pupils with telephoto and retro focus lenses and have shown that they are quite asymmetric. This asymmetry ties back into the position of the principal planes in these lenses.

It is easiest to start by discussing a symmetric lens. A symmetric lens will have an equal distribution of positive and negative elements in the lens. This results in the principal planes being near the center of the lens. Place the aperture at the center of the lens and you have a truly symmetric lens.

If there are the same lens elements in front and behind the aperture, the entrance pupil and the exit pupil will be the same size. In this situation the pupils and the principal planes will be in the same position. The entrance pupil will be in the same position as the front principal plane and vice versa.

symettric lens pupils

To add some asymmetry into the lens: A retrofocus lens will have more negative lenses in the front (object side) of the lens than in the rear (detector side). This asymmetry leads to the front and rear principal planes being displaced backward toward the detector. Since there are negative elements in the front of the lens, the aperture will tend to look smaller from the front. From the rear, the aperture will appear larger because of the positive elements (like using a magnifying glass).

The pupils will also change position in relation to the principal planes. The smaller entrance pupil will be out in front of the front principal plane when viewed fron the front - i.e. will move toward the front focal point of the lens. The larger exit pupil, when viewed from the rear of the lens, will move farther away from the viewer and away from the rear focal point of the lens (much easier for me to show this movement with diagrams). Just the opposite occurs with a telephoto lens.

position and size of pupils

P = front principal plane, E = entrance pupil, P' = rear principal plane, E' = exit pupil

The amount that the pupils move in relation to the principal planes is directly related to the pupillary magnification. Referring to the above diagram: The pupillary magnification is two (the exit pupil is twice as large as the entrance pupil, ie. a retrofocus lens). This means that the exit pupil will 2x the focal length behind the rear focal point. The entrance pupil will be 1/2x the focal length behind the front focal point. I left the real aperture position out of this diagram for simplicity (if you could call it that), but it is somwhere in front of the two principal planes not far from the entrance pupil.

In the coming installments I will try to start explaining why these factors actually make a difference in macro photography and why they don't mean diddly for landscape photography. But first, I will have to start getting into aperture related topics like resolution and depth of field to establish definitions for that discussion.