It is the width of the light cone leaving the eyepiece, found by dividing the eyepiece focal length by the focal ratio. Wider than your own pupil and light is wasted; too narrow and the image dims and shows floaters.
Exit pupil is the specification that explains why two people can look through the same instrument and disagree about how bright it is.
The arithmetic
For a telescope, divide the eyepiece focal length by the focal ratio. For binoculars, divide the aperture by the magnification. The answer is a width in millimetres, and it is the diameter of the beam arriving at your eye.
Why too wide is wasteful
If the beam is wider than the pupil of your eye, the surplus lands on the iris and is lost. The telescope collected it; the eye could not accept it. In a reflector, an exit pupil larger than the pupil can also make the shadow of the secondary mirror visible.
Why too narrow is uncomfortable
A very small exit pupil makes the image dim, magnifies the effect of floaters in the eye, and makes positioning the eye fussy. It is the reason very short eyepieces feel harder to use than their focal length alone would suggest.
How to use it
Work it out for the eyepieces you own before buying another. A case whose eyepieces produce a spread of exit pupils is a useful case; one where two of them land in the same place contains a duplicate.
If this page and the manufacturer’s own documentation disagree, follow the documentation. Every aperture, focal length, payload rating, field, eye relief, transmission figure and thread size on this page comes from the maker’s own published figures, with the unit as printed and the source named. None of it is measured here, and no figure is converted into another convention, because a converted number is a different number. And the distinction this page keeps: a magnification is not a property of a telescope. It is the focal length of the tube divided by the focal length of whichever eyepiece is fitted, so it changes every time the eyepiece does, and a figure printed on packaging is a statement about a supplied eyepiece. This page publishes both focal lengths and the aperture, and reproduces no packaging magnification as a specification.
The questions that keep arriving
How wide is a dark-adapted pupil?
Around 7 mm for a young adult in full darkness, and commonly 5 to 6 mm by about fifty — it narrows steadily with age, which is why the same binoculars deliver differently to two people standing side by side. Treat those as orders of magnitude for your own eye rather than as a specification.
Does this apply to binoculars too?
Yes, and it is easier there: divide the aperture by the magnification. A pair whose exit pupil exceeds your own pupil is throwing away the surplus, which is why very large exit pupils suit dark skies and young eyes.
Last reviewed 17 September 2026