Work out the magnification of any telescope and eyepiece pair, with or without a Barlow lens. You also get the exit pupil, true field of view, useful magnification range, focal ratio, resolving power and a table for a whole set of eyepieces.
About 7 mm for young eyes, 5–6 mm for older observers
Enter your values to see the result.
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How to use the Telescope Magnification Calculator
Enter the telescope focal length and aperture (printed on the tube or in the manual).
Enter the eyepiece focal length (printed on the eyepiece) and its apparent field.
Choose a Barlow or reducer if you use one.
Read the magnification, exit pupil and field of view.
Use the eyepiece table to plan a set covering low, medium and high power.
Telescope formulas
Quantity
Formula
Example (1200 mm, 203 mm, 25 mm 52° eyepiece)
Magnification
M = Ftelescope × Barlow / Feyepiece
48×
Exit pupil
D / M
4.23 mm
True field
apparent field / M
1.08°
Focal ratio
F / D
f/5.9
Max useful power
≈ 2 × D (mm)
406×
Min useful power
D / eye pupil
29× (7 mm pupil)
Dawes limit
116 / D (mm) arcsec
0.57″
Choosing magnification by target
Exit pupil
Power per inch of aperture
Good for
5–7 mm
3.6–5×
Large nebulae, star fields, finding objects
3–4 mm
6–8×
Open clusters, larger galaxies
2–3 mm
8–13×
Galaxies, globular clusters, planetary nebulae
1–2 mm
13–25×
Moon, planets, double stars
0.5–1 mm
25–50×
Planetary detail and close doubles in steady air
The atmosphere ("seeing") usually caps practical magnification at around 250–350× no matter how large the telescope.
Frequently asked questions
How do you calculate telescope magnification?
Divide the telescope's focal length by the eyepiece's focal length: M = F / f. A 1200 mm telescope with a 25 mm eyepiece gives 48×. A 2× Barlow doubles the effective focal length, so it doubles the magnification.
What is the maximum useful magnification?
A common rule is about 2× per millimetre of aperture (50× per inch). A 203 mm (8-inch) telescope tops out around 400×, and in practice the atmosphere often limits it to 200–300×.
What is the exit pupil?
The diameter of the beam of light leaving the eyepiece: aperture ÷ magnification (or eyepiece focal length ÷ focal ratio). About 2–4 mm suits most deep-sky objects, 0.5–1.5 mm is for planets, and more than your eye's pupil (about 7 mm) wastes light.
How do I find the true field of view?
Divide the eyepiece's apparent field of view by the magnification. A 52° eyepiece at 48× shows about 1.08°, roughly two full Moons side by side.
Does higher magnification mean a better telescope?
No. Aperture determines how much light is gathered and how much detail can be resolved. Magnification only enlarges the image, and beyond the useful limit it just makes a dimmer, blurrier view.
What does the Dawes limit mean?
It is the smallest separation (in arcseconds) at which two equal stars can be split: 116 / D with D in mm. An 8-inch telescope resolves about 0.57″ in perfect seeing.
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