Exit pupil

Single lens imaging with the aperture stop. The exit pupil is the image of the aperture stop formed by the optics behind it, and the location and size of the pupil are determined by chief rays and marginal rays.
The image side of the lens of an SLR camera; the exit pupil is the light area in the middle of the lens.

In optics, the exit pupil is a virtual aperture in an optical system. Only rays which pass through this virtual aperture can exit the system. The exit pupil is the image of the aperture stop in the optics that follow it. In a telescope or compound microscope, this image is the image of the objective element(s) as produced by the eyepiece. The size and shape of this disc is crucial to the instrument's performance, because the observer's eye can see light only if it passes through the aperture. The term exit pupil is also sometimes used to refer to the diameter of the virtual aperture. Older literature on optics sometimes refers to the exit pupil as the Ramsden disc, named after English instrument-maker Jesse Ramsden.