# Exit pupil

The width of the beam of light leaving the eyepiece: aperture ÷ magnification. It sets how bright and sharp the view looks.

Hold a telescope up to the daylight and you'll see a small bright disk floating behind the eyepiece: that's the exit pupil. A big one (5–7 mm) gives the brightest, widest views for faint nebulae — but never more than your eye's pupil (about 7 mm when young, 5–6 mm later in life). A small one (0.5–1 mm) gives high power for planets.

Exit pupil is the observer's secret weapon: the same exit pupil gives the same surface brightness in any telescope, so experienced observers choose eyepieces by it.

**In AstroPilot:** AstroPilot picks eyepieces by the exit pupil each kind of object needs, and the gear page shows which sweet spots your kit covers.

**Related:** [Magnification](https://astropilot.space/glossary/magnification.md), [Aperture](https://astropilot.space/glossary/aperture.md), [Sweet spots](https://astropilot.space/glossary/sweet-spot.md), [Dark adaptation](https://astropilot.space/glossary/dark-adaptation.md), [Surface brightness](https://astropilot.space/glossary/surface-brightness.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/exit-pupil