Online Tool Store
Telescope Magnification Calculator
Calculate magnification, exit pupil, and true field of view for an eyepiece and telescope pairing, including the useful magnification limit.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Telescope magnification
Enter your telescope and eyepiece specs.
Magnification
Exit pupil
True field
Useful maximum for this aperture: about 400x on an excellent night.
How the Telescope Magnification Calculator works
- Enter the telescope's focal length and aperture, both usually printed on the tube.
- Add the eyepiece focal length and its apparent field of view.
- Check the exit pupil — below about 0.5 mm the image dims and floaters in your eye become visible.
The method
Magnification is the ratio of the two focal lengths, and exit pupil is the aperture divided by that magnification.
magnification = telescope focal length ÷ eyepiece focal length; exit pupil = aperture ÷ magnification
1200/10 = 120x, and 200/120 = 1.67 mm exit pupil. True field is the eyepiece's apparent field divided by magnification: 68/120 = 0.57°.
FAQ
What is the maximum useful magnification?
Roughly 2x the aperture in millimetres on an excellent night, and often far less. Atmospheric seeing usually limits you well before the optics do, which is why very high magnifications mostly produce a bigger blur.
Why does exit pupil matter?
It sets image brightness and comfort. Too large and light spills past your eye's pupil; too small and the view dims while imperfections in your own eye become visible. Around 2-3 mm suits most deep-sky viewing.
Why does my true field shrink at high magnification?
Because true field is the eyepiece's apparent field divided by magnification. Doubling the power halves the sky you can see, which is why finding objects starts at low power.
How we compare
| Feature | Online Tool Store | A graphing calculator | A stats package |
|---|---|---|---|
| Exit pupil included | ✓ | ✗ | ✓ |
| True field of view | ✓ | Manual | ✓ |
| No account | ✓ | ✓ | ✗ |
| Accounts for seeing conditions | ✗ | Experience | ✗ |
Telescope Magnification Calculator reports exit pupil and true field, which decide whether a magnification is actually usable rather than just available.