The Telescope Magnification & Exit Pupil Calculator works out how much a scope-and-eyepiece pairing enlarges the sky, plus the supporting numbers that decide whether that magnification is actually usable. Switch to focal-ratio mode to read the f-number of the telescope on its own.
Magnification = telescope focal length ÷ eyepiece focal length, so a 1200 mm scope with a 10 mm eyepiece gives 120×. Exit pupil = aperture ÷ magnification, the width in millimetres of the light beam reaching your eye; focal ratio = focal length ÷ aperture; and the maximum useful magnification is roughly 2× the aperture in millimetres (about 50× per inch). If you enter the eyepiece's apparent field of view, the true field of view = apparent field ÷ magnification.
All lengths are entered in millimetres and fields in degrees. A large exit pupil (5–7 mm) gives bright, wide low-power views, while pushing past the maximum useful power just yields a dim, blurry image. Everything runs entirely in your browser with no uploads, so you can cycle through eyepieces instantly.
Frequently asked questions
How do I calculate telescope magnification?
Divide the telescope's focal length by the eyepiece's focal length. A 1200 mm telescope with a 10 mm eyepiece yields 1200 ÷ 10 = 120×. A shorter eyepiece gives higher magnification.
What is exit pupil and why does it matter?
Exit pupil = aperture ÷ magnification, the diameter of the light cone leaving the eyepiece. Below about 0.5 mm the view is dim and star-poor; above roughly 7 mm light is wasted beyond your dark-adapted pupil.
What is the maximum useful magnification?
About 2× the aperture in millimetres, or roughly 50× per inch — so 300× on a 150 mm scope. Beyond this the image only gets bigger and dimmer without revealing more real detail, and atmospheric seeing usually limits you first.
How do I get the true field of view?
Enter the eyepiece's apparent field of view (often 50–68°) and the tool divides it by the magnification. For example a 52° eyepiece at 120× shows a true field of about 0.43°, a little under one Moon diameter.