Telescope Magnification Calculator
Magnification, exit pupil and useful limits.
Show detailsHide details
The right magnification depends on your telescope and eyepiece — not just a big number on the box. Enter your focal lengths and this calculator gives the true magnification, plus the exit pupil and the sensible magnification limits for your aperture.
Loading calculator...
Telescope Magnification — The Quick Answer
Magnification is the telescope's focal length divided by the eyepiece's focal length. A 1000 mm telescope with a 10 mm eyepiece gives 100×. A Barlow lens multiplies that further.
Shortcut
Magnification = telescope focal length ÷ eyepiece focal length × Barlow
Magnification from a 1000 mm telescope
| Eyepiece | Magnification |
|---|---|
| 25 mm | 40× |
| 15 mm | 67× |
| 10 mm | 100× |
| 6 mm | 167× |
| 10 mm + 2× Barlow | 200× |
More magnification isn't always better. The useful maximum is about 2× the aperture in millimetres — push past it and you just magnify blur, not detail.
How It Works
The Formulas
Optics in a nutshell
Formula
Magnification = scope FL ÷ eyepiece FL × Barlow Exit pupil = aperture ÷ magnification Max useful ≈ 2 × aperture (mm)
Variables
Telescope focal length
The focal length of the telescope in mm — often printed near the focuser or in the specs.
Eyepiece focal length
The focal length of the eyepiece in mm. Smaller eyepieces give higher magnification.
Beam width
The width of the light beam leaving the eyepiece. Below about 0.5 mm the view gets dim; above 7 mm you waste light.
Note: The useful magnification range runs from about aperture ÷ 7 (lowest) to aperture × 2 (highest, in mm). Atmospheric steadiness usually caps practical magnification around 250–300× regardless of the telescope.
Worked Example: A 1000 mm Scope at Different Powers
Magnification, a Barlow, and the useful ceiling
Divide the focal lengths
1000 mm telescope ÷ 10 mm eyepiece = 100×.
Add a Barlow if used
A 2× Barlow doubles it: 100× → 200×.
Check it's useful
For a 100 mm aperture, the useful maximum is about 2× = 200×, so 200× sits right at the sensible ceiling.
Reference Guide
| Eyepiece | Magnification | Exit pupil (100 mm) |
|---|---|---|
| 25 mm | 40× | 2.5 mm |
| 15 mm | 67× | 1.5 mm |
| 10 mm | 100× | 1.0 mm |
| 6 mm | 167× | 0.6 mm |
| 10 mm + 2× Barlow | 200× | 0.5 mm |
Choosing the Right Power
The best magnification depends on the target and the sky
Bright, wide views for star clusters, nebulae, and finding targets before you zoom in.
Best for: Deep-sky objects and sweeping the sky
Tighter views for the Moon, planets, and double stars — useful right up to the limit for your aperture.
Best for: Lunar and planetary detail
Past about 2× the aperture in mm you magnify blur, not detail, and the image dims. Atmospheric steadiness often caps you lower still.
Best for: Knowing when more power stops helping
Common Mistakes
Chasing the 'biggest number'
Box claims like '600×' are marketing. Useful magnification is capped near 2× the aperture in mm — beyond that you just enlarge a fuzzy, dim image.
Ignoring aperture
Magnification without enough aperture gives a dim, soft view. A small scope at high power can't out-resolve a larger one; aperture sets the real detail limit.
Forgetting the sky's steadiness
Atmospheric turbulence usually caps practical magnification around 250–300× on all but the steadiest nights, whatever the telescope allows.
Overlooking exit pupil
Too small (under ~0.5 mm) and the view dims and shows floaters; too large (over ~7 mm) wastes light your eye can't take in. Aim for roughly 1–4 mm.
Last updated: October 8, 2026
Creators
Computer Scientist & Developer Tools Author
Reviewers
Technical Reviewer
Table of Contents
Quick Facts
Related Tools
Privacy Guaranteed
Your data never leaves your browser. All calculations are 100% private.
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 20 mm eyepiece gives 1200 ÷ 20 = 60×. A Barlow lens multiplies the result (a 2× Barlow would make it 120×).
What is the maximum useful magnification?
About 2× the aperture in millimetres — so roughly 400× for a 200 mm telescope. Beyond that, the image gets dim and fuzzy without revealing more detail. In practice, atmospheric turbulence often limits you to 250–300×.
What is exit pupil and why does it matter?
Exit pupil is the width of the light beam leaving the eyepiece, equal to aperture ÷ magnification. Too small (under ~0.5 mm) and the view is dim; too large (over ~7 mm) and your eye can't use all the light. A comfortable range is about 1–4 mm.
Is higher magnification always better?
No. Low power gives bright, wide views ideal for star clusters and finding objects; high power suits the Moon and planets. The best magnification depends on the target, your aperture, and the steadiness of the sky.