# Telescope Magnification Calculator

URL: https://speedycalculator.com/calculators/physics/lens-magnification/
Last updated: 2026-10-25

> The telescope magnification calculator divides the telescope’s focal length by the eyepiece focal length to give magnification, and estimates the exit pupil and useful maximum magnification.

## Inputs

- Telescope focal length (mm)
- Eyepiece focal length (mm)
- Aperture (mm)

## Results

- Magnification
- Exit pupil
- Useful maximum magnification
- Focal ratio

## Formula

```
Magnification = telescope focal length ÷ eyepiece focal length
Exit pupil = aperture ÷ magnification
```

## Explanation

Higher magnification is not always better: atmospheric conditions and aperture limit useful power to roughly twice the aperture in millimetres.

### How to use the Telescope Magnification Calculator

- **Telescope focal length**
- **Eyepiece focal length**
- **Aperture**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Divide the two focal lengths for magnification; divide aperture by magnification for exit pupil.

### Inputs explained

FieldWhat to enterTelescope focal lengthIn mmEyepiece focal lengthIn mmApertureIn mm

### What each result means

- **Magnification** = [Telescope focal length] ÷ [Eyepiece focal length]
- **Exit pupil** = [Aperture] ÷ ([Telescope focal length]÷[Eyepiece focal length])
- **Useful maximum magnification** = 2 × [Aperture]
- **Focal ratio** = [Telescope focal length] ÷ [Aperture]

### 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)Magnification (×)60024900361,200481,500601,800722,40096

## Worked example

Inputs — Telescope focal length: 1,200 mm; Eyepiece focal length: 25 mm; Aperture: 200 mm.

Results — **Magnification = 48 ×**; Exit pupil = 4.17 mm; Useful maximum magnification = 400 ×; Focal ratio = 6.

## FAQ

**What exit pupil is best?**
About 2–7 mm; your eye’s pupil limits the largest useful exit pupil.

**Does a Barlow lens change this?**
Yes, a 2× Barlow doubles the magnification.

**What formula does the Telescope Magnification Calculator use?**
Magnification = telescope focal length ÷ eyepiece focal length.