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Astronomy Telescope Magnification Calculator

Calculate telescope magnification, exit pupil, and true field of view. Enter focal length and eyepiece specs for refractors and reflectors.

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Inputs that matter
Objective Focal Length, Eyepiece Focal Length, Eyepiece Apparent FOV
Output to expect
Telescope Magnification Calculator
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Magnification

Magnification equals telescope focal length divided by eyepiece focal length. A 1200mm scope with a 25mm eyepiece gives 48× magnification.

Magnification = Telescope FL / Eyepiece FL

Exit Pupil

Exit pupil is the diameter of the light beam leaving the eyepiece. It equals aperture divided by magnification. Pupils dilate to ~7mm dark-adapted; larger exit pupils waste light.

Exit pupil = Aperture / Magnification (mm)

True Field of View

True field equals apparent field of the eyepiece divided by magnification. A 50° apparent field eyepiece at 100× gives 0.5° true field (about one lunar diameter).

True FOV = Apparent FOV / Magnification

Updated: July 2026

Example Scenarios

1200mm FL / 25mm = 48× magnification. Exit pupil = 203mm / 48 = 4.2mm. With 50° AFOV eyepiece, true field = 1.04° — fits the full Moon.

Same scope with 5mm eyepiece: 240× magnification, 0.85mm exit pupil. Best for steady seeing on Jupiter and Saturn; too dim for deep-sky.

80mm f/5 refractor (400mm FL) with 24mm 68° eyepiece: 17× mag, 4.7mm exit pupil, 4° true field for sweeping star fields.

Common Mistakes to Avoid

Using maximum magnification for all targets

Low magnification provides wider field and brighter image for finding objects and large nebulae. Reserve high power for planets, double stars, and lunar detail in steady air.

Ignoring exit pupil for dark-adapted observing

Exit pupil larger than your dark-adapted eye (typically 5–7mm) wastes light at the eyepiece. At 100× on a 4-inch scope, exit pupil is 1mm — fine for planets but dim for faint galaxies.

FAQ

Planets typically look best at 25× to 50× per inch of aperture. An 8-inch scope performs well at 200–400× in steady seeing. Start low and increase until image brightness and sharpness degrade.

Exit pupils of 2–4mm work well for most deep-sky observing. At 2mm you maximize contrast; at 4mm you balance brightness and magnification for nebulae and galaxies.

Yes. Empty magnification occurs when the image is enlarged beyond what atmospheric seeing or aperture supports. Rule of thumb: max useful magnification ≈ 50× per inch of aperture (200× for 4-inch).

A 2× Barlow doubles effective magnification by halving effective eyepiece focal length. A 25mm eyepiece with 2× Barlow acts like a 12.5mm eyepiece for magnification purposes.

Prime focus photography uses the scope alone — effective "magnification" relative to naked eye is roughly focal length / 7mm. With eyepiece projection, multiply by eyepiece magnification factor.

About Astronomy Telescope Magnification Calculator

Choosing the right eyepiece depends on magnification, exit pupil size, and true field of view. This calculator computes all three from your telescope focal length and eyepiece specifications so you can match power to target and sky conditions.