The exit pupil is the width of the beam of light leaving your eyepiece, measured in millimetres. It decides how bright the view looks and whether a given magnification is actually usable. Match it to your eye and the target, not just to the biggest number on the eyepiece.

Enter your kit

Telescope aperture (mm) The diameter of the main mirror or lens, e.g. 200

Telescope focal length (mm) Printed near the focuser, e.g. 1200

Eyepiece focal length (mm) The number on the eyepiece, e.g. 25

Two quick sanity checks come out of this. A dark-adapted adult eye opens to roughly 5 to 7mm, so an exit pupil much larger than that throws away light your eye cannot collect (and, in a reflector, the shadow of the secondary mirror can appear). At the other end, an exit pupil below about 0.5mm gives empty magnification: the image is dim and mushy with no extra detail.

What the numbers mean

Exit pupil is simply aperture divided by magnification, which also equals the eyepiece focal length divided by the telescope's focal ratio. Rough guide by target:

4 to 7mm low power, widest true field, best for large faint nebulae and sweeping the Milky Way.
2 to 4mm the general-purpose sweet spot for most deep-sky objects and star clusters.
0.5 to 2mm high power for the Moon, planets and splitting double stars, seeing conditions permitting.
Below 0.5mm usually too much magnification for the aperture; the view dims without gaining detail.