Telescope Magnification Calculator

Telescope Magnification Calculator. Enter your telescope focal length, aperture, and eyepiece focal length to calculate magnification, maximum useful magnification, exit pupil, and resolving power. The Telescope Magnification Calculator gives you a complete performance breakdown for any scope and eyepiece combination. Also try the find Brewster's Angle with Brewster's Angle Calculator.

mm

The focal length of your telescope's primary lens or mirror, usually printed on the tube.

mm

The diameter of the telescope's main lens or mirror — its most important attribute.

mm

The focal length of the eyepiece, usually printed on the barrel.

°

The apparent (advertised) field of view of the eyepiece in degrees. Used to calculate true field of view.

Apply a Barlow or focal reducer to multiply or reduce effective magnification.

Results

Magnification

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Maximum Useful Magnification

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Exit Pupil Diameter

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Focal Ratio (f/number)

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Resolving Power (Dawes' Limit)

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True Field of View

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Limiting Stellar Magnitude

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How is telescope magnification calculated?

Magnification is calculated by dividing the telescope's focal length by the eyepiece's focal length. For example, a 1000mm telescope with a 25mm eyepiece produces 40× magnification. If you add a 2× Barlow lens, the effective magnification doubles to 80×. See also our calculate Laser Pulse Peak Power.

What is the maximum useful magnification for a telescope?

The theoretical maximum is approximately 2.5× the telescope aperture in millimetres (or 50× per inch of aperture). However, atmospheric seeing conditions typically limit practical maximum magnification to around 250–350×, regardless of aperture. Pushing beyond maximum useful magnification produces a dimmer, blurry image with no added detail.

What is exit pupil and why does it matter?

Exit pupil is the diameter of the beam of light that exits the eyepiece and enters your eye. It equals the telescope aperture divided by the magnification. An exit pupil larger than 7mm may be too wide for the dark-adapted human eye, while one smaller than 0.4mm is impractical as eye floaters and eyelashes interfere with the view.

What is the Dawes' limit (resolving power)?

Dawes' limit describes the smallest angular separation between two stars that a telescope can resolve into two distinct points. It is calculated as 116 ÷ aperture (in mm) and expressed in arcseconds. A larger aperture gives a smaller Dawes' limit value, meaning finer detail can be resolved. You might also find our Lumen Calculator useful.

How does a Barlow lens affect magnification?

A Barlow lens is a diverging lens placed between the telescope and eyepiece that multiplies the effective focal length of the telescope. A 2× Barlow doubles the magnification of any eyepiece used with it, effectively giving you twice as many eyepiece options without buying new glass.

What is true field of view and how is it calculated?

True field of view (TFOV) is the actual sky area visible through the eyepiece, measured in degrees. It is calculated as the eyepiece's apparent field of view (AFOV) divided by the magnification. A wider true field is better for viewing large objects like open clusters and nebulae.

What is focal ratio (f/number) in a telescope?

The focal ratio is the telescope's focal length divided by its aperture. A low f/number (e.g. f/5) means a 'fast' telescope with a wider field of view and brighter images — great for deep-sky objects. A high f/number (e.g. f/10 or more) means a 'slow' telescope better suited to high-power planetary viewing.

How do I choose the right eyepiece for a target?

Use a low-magnification, wide-field eyepiece (long focal length) to locate objects and view large targets like open clusters. Switch to a shorter focal length eyepiece for higher magnification on planets and double stars. Always ensure your chosen magnification stays below the telescope's maximum useful magnification for clear, sharp images.