# Telescope Field of View Calculator

URL: https://speedycalculator.com/calculators/physics/telescope-field-of-view/
Last updated: 2026-10-30

> The telescope field of view calculator finds the true field of view of a telescope and eyepiece from the eyepiece's apparent field and the magnification (focal lengths), plus the sky area seen.

## Inputs

- Telescope focal length (mm, cm, m, in, µm)
- Eyepiece focal length (mm, cm, m, in, µm)
- Apparent field of view (°, rad, arcmin, arcsec, mrad)

## Results

- Magnification
- True field of view
- In arcminutes
- Area of the field of view
- Full Moons across

## Formula

```
M = F_t / f_e,  FOV_true = FOV_apparent / M
```

## Explanation

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

FieldWhat to enterTelescope focal lengthUnits: mm, cm, m, in, µmEyepiece focal lengthUnits: mm, cm, m, in, µmApparent field of viewUnits: °, 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 (°)6002.16679001.44441,2001.08331,5000.86671,8000.72222,4000.5417

## 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.

## FAQ

**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.


## Sources

- [OpenStax – Astronomy 2e](https://openstax.org/details/books/astronomy-2e)
- [OpenStax – University Physics Volume 3](https://openstax.org/details/books/university-physics-volume-3)
- [HyperPhysics (Georgia State University)](http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html)