Telescope Magnification Calculator (FOV, Exit Pupil)
Enter the telescope’s aperture and focal length and the eyepiece’s focal length and apparent field.
Your numbers
Your results
Telescope Magnification Calculator (FOV, Exit Pupil) by TheCalcTools
How the Telescope Magnification Calculator (FOV, Exit Pupil) works
Magnification = telescope focal length ÷ eyepiece focal length (times any Barlow factor). The true field of view is the eyepiece’s apparent field divided by the magnification; the Moon is about 0.5° across.
Exit pupil = aperture ÷ magnification; around 2 to 7 mm suits most viewing. Above about twice the aperture in millimetres, images get dimmer without showing more detail.
The formula
Worked example
Using the default values shown in the calculator:
| Aperture (mm) | 200 |
|---|---|
| Telescope focal length (mm) | 1200 |
| Eyepiece focal length (mm) | 25 |
| Eyepiece apparent field (degrees) | 52 |
| Barlow lens factor | 1 |
- A 1,200 mm telescope with a 25 mm eyepiece magnifies 48×.
- True field 1.08°, exit pupil 4.17 mm.
Result:
| Magnification | 48 |
|---|---|
| True field of view (degrees) | 1.08 |
| Exit pupil (mm) | 4.17 |
| Focal ratio | f/6.0 |
| Highest useful magnification (about 2× aperture in mm) | 400 |
| Resolving power, Dawes limit (arcseconds) | 0.58 |
Frequently asked questions
Is higher magnification better?
No. Atmosphere and aperture limit detail; most good views are at moderate power.
Sources and further reading
Related calculators
More in Hobbies: Photo, Music & Garden · All calculators
This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.