---
title: "Telescope Magnification Calculator | Backyard Orbit"
description: "Magnification, exit pupil, true field, useful power range, resolution and faintest stars for any telescope and eyepiece set, in mm or inches."
url: https://backyardorbit.me/
---

# Telescope magnification & eyepiece calculator

Magnification, exit pupil, true field, the useful range of magnification, resolving power and the faintest stars for your telescope and eyepieces, in millimetres or inches. Every formula is named below.

## What the numbers mean

**Magnification** is the telescope's focal length divided by the eyepiece's (a 1200 mm telescope with a 25 mm eyepiece gives 48×). A Barlow lens multiplies it. If you only know the f-ratio, the focal length is aperture × f-ratio: a 200 mm f/6 has a 1200 mm focal length.

The **exit pupil** is the beam of light leaving the eyepiece: aperture ÷ magnification. If it is wider than the pupil of your eye, some of the light the telescope collects is wasted, which sets the lowest useful magnification. Below about 0.5 mm the view turns dim and soft, which matches the usual top limit of about 2× per millimetre of aperture (50× per inch). On most nights the air limits you well before that.

The **true field** is how much sky you see. Dividing the eyepiece's apparent field by the magnification gives a good estimate; if you know the eyepiece's field-stop diameter, field stop ÷ focal length × 57.3 gives the exact figure.

**Resolution** is the closest pair of equal stars the aperture can split: 116″ ÷ aperture in mm (Dawes) or 138″ ÷ aperture (Rayleigh, for yellow-green light). The **faintest stars** figure starts from what John Bortle reported for a 12.5-inch telescope under each class of sky and scales it to your aperture. Treat it as a rough guide: eyesight, experience and steady air all change it.

## Sources

- Tele Vue Optics, [Choosing Eyepieces (Basic)](https://televue.com/mobile/TV5_page.asp?id=79): magnification = telescope focal length ÷ eyepiece focal length; exit pupil = eyepiece focal length ÷ f-ratio; true field = field stop ÷ telescope focal length × 57.3.
- Al Nagler, [Choosing Your Telescope's Magnification](https://www.televue.com/engine/TV3b_page.asp?id=102), *Sky & Telescope*, May 1991: exit pupil = aperture ÷ magnification; true field ≈ apparent field ÷ magnification; 50× per inch (a 0.5 mm exit pupil) as the high end; the 7 mm dark-adapted pupil; 20–30× per inch for planetary detail.
- W. R. Dawes, Catalogue of Micrometrical Measurements of Double Stars, *Memoirs of the Royal Astronomical Society* 35 (1867) 137: 4.56″ ÷ aperture in inches, which is 116″ ÷ aperture in mm.
- HyperPhysics (R. Nave, Georgia State University), [The Rayleigh Criterion](https://hyperphysics.gsu.edu/hbase/phyopt/Raylei.html): θ = 1.22 λ ÷ D, which at 550 nm is 138″ ÷ aperture in mm.
- R. Lazar, J. Degen, A.-S. Fiechter, A. Monticelli, M. Spitschan, Regulation of pupil size in natural vision across the human lifespan, *Royal Society Open Science* 11 (2024) 191613, [doi:10.1098/rsos.191613](https://doi.org/10.1098/rsos.191613): the dark-adapted pupil shrinks by about 0.41 mm per decade of age (8.13 mm − 0.408 mm × age in decades), used for the age estimates.
- John E. Bortle, [Introducing the Bortle Dark-Sky Scale](https://skyandtelescope.org/wp-content/uploads/BortleDarkSkyScale.pdf), *Sky & Telescope*, February 2001, pp. 126–129: the faintest stars a 32-cm (12.5-inch) reflector shows in each class of sky. We scale them by 5 × log₁₀(your aperture ÷ 320 mm), the magnitude change for the difference in light grasp.
- NASA Moon Fact Sheet: the full Moon's average apparent diameter of 1896″ (31.6′), used for "full-Moon widths".
- American Astronomical Society, [Solar Filters for Optics: Telescopes, Binoculars & Cameras](https://eclipse.aas.org/eye-safety/optics-filters).

## More tools for the backyard

See what fits in the eyepiece, choose binoculars, rate your sky and plan meteor nights.

- [Field of view simulator](https://backyardorbit.me/field-of-view-simulator/): Your eyepiece circle drawn to scale with the Moon, planets, the Orion Nebula, the Pleiades and the Andromeda Galaxy.
- [Binocular calculator](https://backyardorbit.me/binocular-calculator/): Exit pupil, twilight factor, brightness and true and apparent field for any binocular size.
- [Bortle scale guide](https://backyardorbit.me/bortle-scale/): What you can see under each of the nine classes of sky, from inner city to a truly dark site.
- [Meteor shower calendar](https://backyardorbit.me/meteor-shower-calendar/): The major showers of 2026 and 2027 with peak dates, rates, radiants and the Moon at each peak.

## Recommended

Two free services for the online side of the hobby.

Disposable email · no sign-up

### CleanTempMail

Planetarium software, star-chart apps and observing-list sites often want an email address before a free download or trial. CleanTempMail gives you a free temporary inbox straight away, with no sign-up, a random or custom address on hundreds of domains, and messages deleted automatically after 3 days.

Use your real address for anything you pay for.

[Get a temporary inbox](https://cleantempmail.com/)

Disposable cloud browser · free domain lookups

### WebTraceCheck

Buying a used telescope, mount or binoculars through a classified ad or a shop you have never heard of? WebTraceCheck's free DNS and domain lookup shows the details behind a seller's domain, and its disposable cloud browser opens unfamiliar links on an isolated desktop that is deleted after each session.

Nothing to install, with free time every day.

[Try WebTraceCheck](https://webtracecheck.com/)

## Questions

**How do I work out my telescope's magnification?**

Divide the telescope's focal length by the eyepiece's focal length. A 1200 mm telescope with a 25 mm eyepiece gives 48×, and with a 10 mm eyepiece 120×. A 2× Barlow doubles both. Swapping eyepieces is how you change magnification: the telescope itself has no fixed "power".

**What is the highest magnification my telescope can use?**

The usual rule of thumb is about 2× per millimetre of aperture, or 50× per inch: 400× for a 200 mm telescope. Beyond that the exit pupil drops below 0.5 mm and the image gets dim and blurry. Tele Vue notes that the atmosphere usually limits planetary viewing to about 300× whatever the telescope, and Nagler puts the planetary sweet spot at 20–30× per inch.

**What is exit pupil, and why does it matter?**

It is the width of the beam of light leaving the eyepiece: aperture divided by magnification, or eyepiece focal length divided by the f-ratio. If it is wider than your eye's pupil, part of the light misses your eye, so there is no point going lower in magnification. Dark-adapted pupils are often quoted as 7 mm, but they shrink with age: a 2024 study measured about 0.4 mm less per decade of age.

**What is the difference between apparent field and true field?**

The apparent field is how wide the circle of view looks when you put your eye to the eyepiece, a property of the eyepiece design (often 50° to 82°). The true field is the patch of sky inside that circle. Dividing apparent field by magnification estimates it; the field-stop method is exact when the maker publishes the field-stop diameter.

**What does the f-ratio of a telescope mean?**

It is the focal length divided by the aperture: a 200 mm telescope with a 1200 mm focal length is f/6. With a given eyepiece, a lower f-ratio gives less magnification, a wider true field and a larger exit pupil, because exit pupil = eyepiece focal length ÷ f-ratio.