MasterTool

Telescope Magnification Calculator

Calculate telescope magnification instantly in your browser — the published formula, worked locally with no data sent anywhere.

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What this tool does

Calculate telescope magnification instantly in your browser — the published formula, worked locally with no data sent anywhere. True field uses the standard approximation AFOV ÷ magnification (the field-stop formula is more exact). Exit pupils of roughly 2–5 mm suit most observing; above 7 mm wasted light spills past most eyes' pupils.

How to use the Telescope Magnification Calculator

  1. Enter or select telescope focal length (mm).
  2. Enter or select eyepiece focal length (mm).
  3. Enter or select aperture (mm).
  4. Enter or select eyepiece apparent field of view (degrees).
  5. Read the calculated result; change any measurement to compare alternatives.

Formula

magnification = telescope focal length ÷ eyepiece focal length; exit pupil = aperture ÷ magnification; true field ≈ apparent field ÷ magnification; focal ratio = focal length ÷ aperture.
scopeF
Telescope focal length (mm)
eyeF
Eyepiece focal length (mm)
aperture
Aperture (mm)
afov
Eyepiece apparent field of view (degrees)

True field uses the standard approximation AFOV ÷ magnification (the field-stop formula is more exact). Exit pupils of roughly 2–5 mm suit most observing; above 7 mm wasted light spills past most eyes' pupils.

Worked example

For telescope magnification calculator, the following measurements illustrate the exact method: Telescope focal length (mm): 1000; Eyepiece focal length (mm): 25; Aperture (mm): 150; Eyepiece apparent field of view (degrees): 52.

Inputs

  • Telescope focal length (mm)1000
  • Eyepiece focal length (mm)25
  • Aperture (mm)150
  • Eyepiece apparent field of view (degrees)52

Result

  • Magnification (×)40
  • Exit pupil (mm)3.75
  • True field of view (degrees)1.3
  • Focal ratio (f/)6.67
  • Useful magnification guideAbout 150×–300× for this aperture; beyond ~2× per mm of aperture the image grows but gains no detail

Results explained

Magnification (×)
Magnification (×) from the formula above. True field uses the standard approximation AFOV ÷ magnification (the field-stop formula is more exact). Exit pupils of roughly 2–5 mm suit most observing; above 7 mm wasted light spills past most eyes' pupils.
Exit pupil (mm)
Exit pupil (mm) from the formula above. True field uses the standard approximation AFOV ÷ magnification (the field-stop formula is more exact). Exit pupils of roughly 2–5 mm suit most observing; above 7 mm wasted light spills past most eyes' pupils.
True field of view (degrees)
True field of view (degrees) from the formula above. True field uses the standard approximation AFOV ÷ magnification (the field-stop formula is more exact). Exit pupils of roughly 2–5 mm suit most observing; above 7 mm wasted light spills past most eyes' pupils.
Focal ratio (f/)
Focal ratio (f/) from the formula above. True field uses the standard approximation AFOV ÷ magnification (the field-stop formula is more exact). Exit pupils of roughly 2–5 mm suit most observing; above 7 mm wasted light spills past most eyes' pupils.
Useful magnification guide
Useful magnification guide from the formula above. True field uses the standard approximation AFOV ÷ magnification (the field-stop formula is more exact). Exit pupils of roughly 2–5 mm suit most observing; above 7 mm wasted light spills past most eyes' pupils.

Frequently asked questions

magnification = telescope focal length ÷ eyepiece focal length; exit pupil = aperture ÷ magnification; true field ≈ apparent field ÷ magnification; focal ratio = focal length ÷ aperture.

True field uses the standard approximation AFOV ÷ magnification (the field-stop formula is more exact). Exit pupils of roughly 2–5 mm suit most observing; above 7 mm wasted light spills past most eyes' pupils.

Every field is labelled with its unit — enter values in exactly the labelled unit and read the result in the labelled output unit. Fields with a unit symbol also support the site-wide imperial/metric switch, which converts before the formula runs.

It uses double-precision arithmetic and the published constants and formulas named on this page. The last displayed digit may be rounded, and real conditions can differ from the idealized model described in the assumptions.

Yes. Every calculation runs entirely in your browser; nothing you enter is uploaded, stored or shared.