Work out the exit pupil each eyepiece gives in your telescope and aim for a spread rather than a cluster. Wide exit pupils suit faint extended objects, narrow ones suit planets, and the extremes of the range are where comfort fails.
Choosing by focal length alone is why so many cases contain duplicates. Choosing by exit pupil produces a case that covers the sky.
The arithmetic
Divide the eyepiece focal length by the telescope’s focal ratio. The answer is a width in millimetres — the diameter of the beam of light arriving at your eye.
The wide end
Large exit pupils give bright, low-power, wide views that suit large faint objects and sweeping. Beyond your own pupil width the surplus is lost, and in a reflector the shadow of the secondary can become visible.
The narrow end
Small exit pupils give high magnification and a dim, fussy image where floaters intrude. This is where planetary detail lives, and where comfort decides how long you look.
How to build a case with it
Aim for a spread across the range rather than a cluster in the middle. Two eyepieces that produce nearly the same exit pupil in your telescope are one eyepiece, whatever their barrels say.
The questions that keep arriving
What is the widest useful exit pupil?
Whatever your own dark-adapted pupil admits — around 7 mm for a young adult, commonly 5 to 6 mm by about fifty, and narrowing steadily after that. Beyond it, light leaves the eyepiece and lands on the iris. It is the smaller of the two diameters that decides, always.
And the narrowest?
The point where the image becomes too dim and floaters become distracting, which is personal too. Very short eyepieces in slow telescopes reach it easily.
Last reviewed 17 September 2026