Telescope Magnification Calculator
Calculate telescope magnification from focal lengths.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Enter the focal length of the objective and eyepiece in millimeters. The calculator shows the magnification and approximate field of view.
About Telescope Magnification
Telescope magnification is determined by the ratio of the objective focal length to the eyepiece focal length. Longer objectives or shorter eyepieces increase magnification. However, higher magnification narrows the field of view and reduces image brightness. Practical telescopes balance magnification with aperture, focal ratio, and seeing conditions. Understanding magnification is essential for choosing eyepieces and planning astronomical observations.
Frequently Asked Questions
How is telescope magnification calculated?
Magnification equals the objective focal length divided by the eyepiece focal length: M = fo/fe. A 1000 mm objective with a 25 mm eyepiece gives 40x magnification.
Is higher magnification always better?
No. Higher magnification reduces the field of view and brightness. It also amplifies atmospheric distortion and vibrations. Useful magnification is limited by the objective diameter.
What is the maximum useful magnification?
A common rule is 2x per millimeter of aperture. A 100 mm telescope has a useful limit of about 200x. Beyond this, the image becomes dim and blurry.