Telescope Magnification Calculator

Divide the telescope focal length by the eyepiece focal length. A 1000 mm scope with a 10 mm eyepiece gives 1000 divided by 10, which is 100x. Swap in a shorter eyepiece for more power and a longer one for less.

A common rule of thumb is about 2 times the aperture in millimetres. A 100 mm aperture tops out near 200x, and a 150 mm aperture near 300x. Push past that and the image grows but only gets dimmer and blurrier, since no extra detail is being gathered.

The exit pupil is the width of the light cone leaving the eyepiece, equal to the aperture divided by the magnification. A 100 mm aperture at 100x gives a 1 mm exit pupil. Larger exit pupils look brighter, but going wider than your own dilated pupil, roughly 7 mm, wastes light.

It is the telescope focal length divided by the aperture. A 1000 mm scope with a 100 mm aperture is f/10, and a 1200 mm scope with a 150 mm aperture is f/8. Lower f-numbers give brighter, wider fields and are called fast, while higher ones are slower and better for planets.

Aperture, not eyepiece choice, sets the real limit on detail. Beyond the maximum useful power the image spreads over more of your view without adding resolution, so it turns dim and soft. Atmospheric turbulence often caps the usable power even lower on a given night.

Yes. Enter focal lengths and aperture in millimetres, which is how manufacturers print them. Magnification and maximum useful power are plain multipliers, the exit pupil comes out in millimetres, and the focal ratio is a unitless number written after f/.

No. Magnification only needs the two focal lengths. The aperture is optional and, when supplied, unlocks the focal ratio, exit pupil, and maximum useful power. Leave it blank and you still get the magnification.

It is a guideline, not a hard law. The 2x aperture figure assumes steady skies and clean optics. Sharp planetary nights may allow a little more, while poor seeing, haze, or a low target often make far less the practical ceiling.

It runs entirely in your browser. The optics math is plain JavaScript, so your focal lengths and aperture never leave your computer and nothing is stored on a server.