出射[光]瞳在光学系统上是光圈的真正直径，只有穿过这个真正口径的光线可以离开系统。出射[光]瞳在光学上是跟随在影像的口径中止位置之后，这真正开口和直径的意义是相同的。

对望远镜，出射[光]瞳的直径可以用目镜的焦长除以望远镜的焦比（F值）来计算；更简单的算法是物镜的直径除以放大倍数。

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 this tiny 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.