Telescope Magnification & Eyepiece Calculator

Enter your telescope and the eyepieces you own to see the magnification, exit pupil and true field of view each one gives, along with your scope's focal ratio, useful-magnification limits and resolution. Everything is worked out in your browser, with a plain-language note on what each combination is best for.

Your setup

Aperture (mm) The main mirror or lens diameter. Telescope focal length (mm) Often printed on the tube, e.g. 1200. Eyepiece focal lengths (mm) One or more, separated by commas, e.g. 32, 25, 15, 9, 6. Eyepiece apparent field (deg) About 50 for a Plossl, 68-82 for wide-angle. Barlow / reducer 1 for none, 2 for a 2x Barlow, 0.63 for a reducer.

CycleCalcs.comEnter your telescope's aperture and focal length to begin.

How the numbers are worked out

All of this is simple optics, no astronomy required. From your telescope's aperture (the diameter of the main lens or mirror) and its focal length, plus each eyepiece's focal length:

Magnification = telescope focal length ÷ eyepiece focal length (multiplied by a Barlow or reducer factor if you use one). A 1200 mm scope with a 10 mm eyepiece gives 120×.
Exit pupil = aperture ÷ magnification (equivalently, eyepiece focal length ÷ focal ratio). It is the width of the light beam reaching your eye; keep it under about 7 mm so your own pupil can take in all the light.
True field of view = eyepiece apparent field ÷ magnification. It is how much sky you actually see, in degrees.
Focal ratio (the f/number) = focal length ÷ aperture. Lower numbers give brighter, wider fields; higher numbers reach high power more easily.

The maximum useful magnification is about twice the aperture in millimeters; beyond it the image just gets dimmer and softer (empty magnification), and unsteady air usually limits you sooner. The lowest useful magnification is where the exit pupil reaches roughly 7 mm. The Dawes limit, 116 divided by the aperture in millimeters, is the finest pair of stars the aperture can split, in arcseconds. For the faintest star this scope can reach under your own sky, including the effect of light pollution, open the Can I see it tonight? tab above.

Planning a session? See what is up with Today in the Sky, the Planet Parade Tracker, or whether you are in the path of the next solar eclipse.

Frequently asked questions

What magnification can my telescope handle?

The practical ceiling is about twice the aperture in millimeters (for example about 200x for a 100 mm scope, or 400x for a 200 mm). That is an optical limit; in real air, atmospheric steadiness usually caps useful magnification well below it. Past the limit the image only grows bigger, dimmer and fuzzier, which is called empty magnification.

What is exit pupil, and what is a good value?

The exit pupil is the width of the beam of light leaving the eyepiece, equal to the aperture divided by the magnification (the same as the eyepiece focal length divided by the focal ratio). About 0.5 to 1 mm suits the Moon and planets, 2 to 3 mm is a versatile all-rounder, and 4 to 7 mm gives the brightest wide-field views. Above about 7 mm your eye's pupil cannot open wide enough to catch all the light, so some aperture is wasted.

What eyepiece should I use for planets, and what for nebulae?

For the Moon, planets and double stars use a short eyepiece for high magnification and a small exit pupil of about 0.5 to 1 mm. For large nebulae, star clusters and the Milky Way use a long eyepiece for low magnification, a wide true field, and a larger exit pupil of 4 to 6 mm that keeps the view bright.

What is true field of view?

True field of view is how much sky you actually see through the eyepiece, in degrees. It equals the eyepiece's apparent field of view divided by the magnification. A wider true field makes targets easier to find and frames large objects better; higher magnification narrows it.

Why can't I just use the highest magnification?

More magnification spreads the same light over a larger image, so the view dims, and it also magnifies the blurring of the atmosphere and any shake. Beyond about twice the aperture in millimeters there is no extra detail to reveal, only a bigger, softer, fainter image. Most observing is done at low to medium power.

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