Higher magnification is not always better: atmospheric conditions and aperture limit useful power to roughly twice the aperture in millimetres.
How to use the Telescope Magnification Calculator
- Telescope focal length
- Eyepiece focal length
- Aperture
Results update as you type, and every field accepts the units shown in its menu.
How it works
Divide the two focal lengths for magnification; divide aperture by magnification for exit pupil.
Inputs explained
| Field | What to enter |
|---|---|
| Telescope focal length | In mm |
| Eyepiece focal length | In mm |
| Aperture | In mm |
What each result means
- Magnification = [Telescope focal length] ÷ [Eyepiece focal length]
- Exit pupil = [Aperture] ÷ ([Telescope focal length]÷[Eyepiece focal length])
- Useful maximum magnification = 2 × [Aperture]
- Focal ratio = [Telescope focal length] ÷ [Aperture]
Example values
How the main result changes when Telescope focal length changes and everything else stays at the example values above:
| Telescope focal length (mm) | Magnification (×) |
|---|---|
| 600 | 24 |
| 900 | 36 |
| 1,200 | 48 |
| 1,500 | 60 |
| 1,800 | 72 |
| 2,400 | 96 |
Formula
Magnification = telescope focal length ÷ eyepiece focal lengthExit pupil = aperture ÷ magnificationWorked example
Inputs — Telescope focal length: 1,200 mm; Eyepiece focal length: 25 mm; Aperture: 200 mm.
Results — Magnification = 48 ×; Exit pupil = 4.17 mm; Useful maximum magnification = 400 ×; Focal ratio = 6.
Frequently asked questions
What exit pupil is best?
About 2–7 mm; your eye’s pupil limits the largest useful exit pupil.
Does a Barlow lens change this?
Yes, a 2× Barlow doubles the magnification.
What formula does the Telescope Magnification Calculator use?
Magnification = telescope focal length ÷ eyepiece focal length.