Telescope Magnification Calculator
Magnification, field of view and the limits your aperture actually sets.
Magnification is focal length of the telescope divided by focal length of the eyepiece.
How the telescope magnification calculator works
Magnification is focal length of the telescope divided by focal length of the eyepiece. It is also the least important number on the box.
Aperture sets what you can actually see. Resolving power follows the Dawes limit — 116 divided by aperture in millimetres, in arcseconds — and no eyepiece can beat it. Past roughly 2× the aperture in millimetres, extra magnification enlarges a blurred image without adding detail, which is why "600× magnification!" on a 60 mm department-store telescope is a marketing claim rather than a capability.
The useful minimum matters too: below about 3.5× per millimetre of aperture the exit pupil grows wider than your own dilated pupil, and you throw away light.
Formula: magnification = telescope focal length / eyepiece focal length
Worked examples
| Inputs | Magnification | Note |
|---|---|---|
| A 200 mm f/6 with a 10 mm eyepiece | 120× | 120× |
| With a 2× Barlow | 240× | 240×, right at the limit |
| Over-magnified | 675× | 675× on a 60 mm — useless |
FAQFrequently asked questions
How do I calculate telescope magnification?
Telescope focal length divided by eyepiece focal length, multiplied by any Barlow.
What is the maximum useful magnification?
About twice the aperture in millimetres. Beyond that you enlarge blur, not detail.
Why do cheap telescopes claim 600×?
Because magnification is easy to print and meaningless without aperture. A 60 mm scope tops out around 120× no matter what eyepiece you fit.
What is the Dawes limit?
116 divided by aperture in millimetres, in arcseconds — the finest detail an aperture can resolve.
What is exit pupil?
The width of the light beam leaving the eyepiece. Above about 7 mm it is wider than a dilated eye and light is wasted.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G and the speed of light
- IAU 2015 Resolution B3 — nominal solar and planetary conversion constants — the astronomical unit, solar mass and planetary radii
- CSIRO Space and Astronomy — Australia's national science agency
Last checked: September 2026. Constants are CODATA 2018 (G, c) and IAU 2015 nominal values (solar and planetary parameters).