Telescope Magnification Calculator

About this calculator

Telescope magnification follows the simple ratio M = telescope focal length / eyepiece focal length, so a 1000 mm focal length telescope paired with a 10 mm eyepiece yields 100× magnification, and swapping in a 25 mm eyepiece drops it to 40×. The same telescope therefore produces a whole range of magnifications simply by changing eyepieces, without altering the telescope's aperture or light-gathering ability.

Amateur astronomers use this calculation to choose the right eyepiece for a given target — low magnification with a wide field for star clusters and galaxies, high magnification for planetary and lunar detail — while keeping in mind that a telescope's aperture sets a practical maximum useful magnification (roughly 50× per inch of aperture), beyond which the image just grows dimmer and blurrier rather than more detailed.