Choosing an eyepiece is a trade-off: higher magnification shows more detail but a smaller patch of sky. The true field tells you whether the whole Moon (about 0.5°) or a large nebula will fit in view.
How to use the Telescope Field of View Calculator
- Telescope focal length
- Eyepiece focal length
- Apparent field of view
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the telescope and eyepiece focal lengths and the eyepiece’s apparent field of view (printed on it, often 50–82°).
Inputs explained
| Field | What to enter |
|---|---|
| Telescope focal length | Units: mm, cm, m, in, µm |
| Eyepiece focal length | Units: mm, cm, m, in, µm |
| Apparent field of view | Units: °, rad, arcmin, arcsec, mrad |
What each result means
- Magnification = [Telescope focal length]÷[Eyepiece focal length]
- True field of view = [Apparent field of view]÷[Magnification]
- In arcminutes = [True field of view]×60
- Area of the field of view = pi×([True field of view]÷2)^2
- Full Moons across = [True field of view]÷0.52
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) | True field of view (°) |
|---|---|
| 600 | 2.1667 |
| 900 | 1.4444 |
| 1,200 | 1.0833 |
| 1,500 | 0.8667 |
| 1,800 | 0.7222 |
| 2,400 | 0.5417 |
Formula
M = F_t / f_e, FOV_true = FOV_apparent / M
Worked example
Inputs — Telescope focal length: 1,200 mm; Eyepiece focal length: 25 mm; Apparent field of view: 52 °.
Results — Magnification = 48 ×; True field of view = 1.0833 °; In arcminutes = 65 ′; Area of the field of view = 0.9218 deg²; Full Moons across = 2.08.
Key terms
- True field of view
- The actual angle of sky visible.
Frequently asked questions
What is the apparent field of view?
The angle the eyepiece's view appears to span to your eye.
How do I see more sky?
Use a longer-focal-length or wider-field eyepiece.
How big is the full Moon?
About 0.5° (31 arcminutes).
Is this formula exact?
It is a good approximation; the field-stop method is more precise.