Telescope magnification calculator

The formula

Magnification = F_scope / F_eyepiece, multiplied by any Barlow factor. A 1200 mm telescope with a 25 mm eyepiece gives 48× — the eyepiece is what changes magnification, not the telescope itself.

Useful magnification limit

The maximum useful magnification is roughly 2× the aperture in millimeters. A 100 mm telescope tops out near 200×. Past that, the image grows but does not show more detail — diffraction and atmospheric turbulence wash it out. Many manufacturers list "675× maximum magnification" on a small refractor. That figure is almost always marketing.

Minimum useful magnification

The lowest magnification you can use without wasting light is about 3.5× the aperture in inches, or aperture (mm) ÷ 7. Below that the exit pupil exceeds your eye's pupil and light spills past your iris.