Telescope Magnification Calculator
Calculate telescope magnification, maximum useful power, exit pupil, and true field of view from focal lengths and aperture.
Results
What is it?
Telescope magnification is determined by dividing the telescope focal length by the eyepiece focal length. The exit pupil, maximum useful magnification, and true field of view are derived from aperture and magnification to reveal practical viewing limits.
How to use
Enter your telescope's focal length and aperture (both found on the telescope body or manual), then enter the eyepiece focal length (printed on the eyepiece barrel). The calculator instantly shows magnification, whether it exceeds the useful maximum, and the resulting exit pupil and field of view.
Example scenario
A 1000 mm focal length, 100 mm aperture refractor with a 25 mm eyepiece gives 40x magnification. Max useful power is 200x. Exit pupil is 2.5 mm — ideal for dark skies. True FOV is 1.25° (assuming 50° AFOV eyepiece), enough to frame open clusters like the Pleiades.
Pro tip
The "rule of thumb" maximum useful magnification is 2x aperture in mm (or 50x per inch). Above that, turbulent atmosphere smears fine detail. An exit pupil above 7 mm wastes light as it exceeds the dark-adapted eye pupil. Optimal exit pupil for deep-sky observing is 3-5 mm; for planets, 0.5-1.5 mm. Try 10 mm, 25 mm, and 6 mm eyepieces to cover low, medium, and high power.