Austrittspupille

Austrittspupille, or exit pupil, is a term from optics describing the image of the aperture stop formed as light passes through an eyepiece and exits the optical system toward the observer’s eye. It is the diameter of the beam of light that leaves the eyepiece and enters the eye.

In telescope and binocular design, the exit pupil is a practical measure of how much light can be delivered to the eye and how easily the observer can position the eye to see the full field of view. The exit pupil size depends on the system’s magnification and aperture. A common approximation is D_exit ≈ D_obj / M, where D_obj is the clear aperture of the objective (or objective lens) and M is the magnification. For refracting telescopes, M = F_t / f_e, with F_t the telescope’s focal length and f_e the eyepiece focal length, yielding D_exit ≈ D_obj × f_e / F_t. For example, a 100 mm (0.1 m) telescope at 20× magnification has an exit pupil of about 5 mm.

The observer’s eye plays a crucial role: the eye’s pupil enlarges in darkness to roughly 5–7 mm. An exit pupil similar in size to the eye pupil allows maximum light transmission; if the exit pupil is much larger, light is not fully utilized, while if it is smaller, image brightness is limited by the exit pupil.

The concept applies broadly to optical designs and is related to how well the eye can be aligned with the beam and how much magnified detail and brightness are perceived.