What is the “exit pupil,” and how does it relate to eyepiece selection?

The exit pupil is the diameter of the beam of light exiting the eyepiece of a telescope and entering the observer’s eye. It determines the brightness and resolution of the viewed image, with larger exit pupils providing brighter but potentially lower-resolution views, and smaller exit pupils offering higher resolution but dimmer images. Selecting an eyepiece with an exit pupil matching the observer’s pupil size optimizes light transmission and viewing comfort, enhancing the observing experience, especially under varying lighting conditions and magnifications.