The uniform circle of light that appears when one looks into the ocular is called the exit pupil.
The exit pupil is essentially the reflected circle of light that comes from the ocular lens of a telescope, microscope, or binoculars. It represents the location where the viewer’s eye aligns with the optical axis of the eyepiece. When observing through an optical instrument, if the eye is positioned correctly relative to the exit pupil, it enables the viewer to see a full and bright field of view.
This phenomenon is crucial for ensuring that the maximum amount of light enters the eye, thus providing a clear and well-illuminated image. If the viewer’s eye is not in line with the exit pupil, parts of the field of view may get cut off, leading to a dimmer and less satisfying observation experience.