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Telescope Magnification Calculator

Magnification, exit pupil and useful limits for a telescope and eyepiece combination.

Enter values

Results update as you type.

Result

Magnification

120 ×

Exit pupil
1.25 mm
Focal ratio
f/8
Maximum useful magnification
300 ×
Lowest useful magnification
21 ×
Resolving power (Dawes)
0.773 arcsec

About the Telescope Magnification Calculator

Magnification, exit pupil and useful limits for a telescope and eyepiece combination.

Also known as: eyepiece magnification, telescope power, exit pupil calculator, max magnification telescope.

How to use this calculator

  1. Enter aperture, telescope focal length, eyepiece focal length, barlow multiplier.
  2. The result updates instantly as you type — press Calculate (or Enter) at any time.
  3. Review the breakdown shown under the main result.
  4. Use Copy result or Share to save the answer or send a link with your inputs.

Formula

Magnification = telescope focal length ÷ eyepiece focal length

Worked example

Using the values pre-filled in the calculator above:

  • Aperture = 150 mm
  • Telescope focal length = 1,200 mm
  • Eyepiece focal length = 10 mm
  • Barlow multiplier = 1

the calculator returns:

  • Magnification: 120 ×
  • Exit pupil: 1.25 mm
  • Focal ratio: f/8
  • Maximum useful magnification: 300 ×