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Astronomy & optics

Telescope magnification calculator

Calculate magnification from telescope and eyepiece focal lengths, with an optional Barlow or reducer factor.

Free & no registration Calculated in your browser
01

Your inputs

The starting values are an example.

Use 1 for no additional optics, or 2 for a 2× Barlow lens, for example.

Decimals: 12.5 · Thousands: 1,234.56. Follow the field instructions for lists.

02

Your result

Enter your values and select Calculate.

Your calculation will appear here.

Your result will include the relevant values and units.

Understand the calculation

Formula and example

How it is calculated

Magnification = telescope focal length × additional factor ÷ eyepiece focal length

Worked example

A telescope with a focal length of 1,000 mm and a 25 mm eyepiece gives 40× magnification without additional optics.

Things to check

  • The calculation gives the geometric magnification factor.
  • Higher magnification makes the image larger. Resolution, brightness and atmospheric stability limit the useful increase in detail.