Telescope Magnification and Exit Pupil Calculator
Fill in the values and the answer updates as you type.
How the Telescope Magnification and Exit Pupil Calculator works
Magnification = telescope focal length ÷ eyepiece focal length. Exit pupil = aperture ÷ magnification, which is the width of the light beam leaving the eyepiece. True field of view = apparent field of view ÷ magnification. An exit pupil of 2 to 5 mm suits most deep-sky viewing. Above about 2× the aperture in millimeters the image only gets dimmer and softer, and the atmosphere usually limits it sooner. Example: with Telescope focal length 1200 mm, Telescope aperture 200 mm, and Eyepiece focal length 25 mm, the result is Magnification = 48; Exit pupil (mm) = 4.17; True field of view (°) = 1.08.
Inputs and results
The Telescope Magnification and Exit Pupil Calculator takes 4 inputs and gives 5 results.
What you enter
- Telescope focal length (mm) — starts at 1200.
- Telescope aperture (mm) — starts at 200.
- Eyepiece focal length (mm) — starts at 25.
- Eyepiece apparent field of view (°) — starts at 52.
What you get
- Magnification
- Exit pupil (mm)
- True field of view (°)
- Telescope focal ratio (f/)
- Maximum useful magnification (2× aperture in mm)
Example: Telescope Magnification and Exit Pupil Calculator with the starting values
With telescope focal length 1200 mm, telescope aperture 200 mm, eyepiece focal length 25 mm and eyepiece apparent field of view 52 °, the calculator returns:
| Magnification | 48 |
|---|---|
| Exit pupil (mm) | 4.17 |
| True field of view (°) | 1.08 |
| Telescope focal ratio (f/) | 6 |
| Maximum useful magnification (2× aperture in mm) | 400 |
Change any value above and the results update straight away.
Telescope Magnification and Exit Pupil Calculator FAQ
How do I use the Telescope Magnification and Exit Pupil Calculator?
Enter telescope focal length, telescope aperture, eyepiece focal length and eyepiece apparent field of view in the fields, then read magnification, exit pupil (mm), true field of view (°) and telescope focal ratio (f/) in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the Telescope Magnification and Exit Pupil Calculator calculate?
Magnification, exit pupil, and true field of view for a telescope and eyepiece. It returns magnification, exit pupil (mm), true field of view (°) and telescope focal ratio (f/) from the 4 values you provide.
What values does the Telescope Magnification and Exit Pupil Calculator start with?
It opens with telescope focal length 1200 mm, telescope aperture 200 mm, eyepiece focal length 25 mm and eyepiece apparent field of view 52 ° so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the Telescope Magnification and Exit Pupil Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the Telescope Magnification and Exit Pupil Calculator for important decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm important decisions with a qualified professional. See the Disclaimer for more.