Telescope Magnification Calculator

About this calculator

This calculator finds a telescope's key optical numbers from its aperture and focal length together with an eyepiece's focal length: magnification = telescope focal length / eyepiece focal length, focal ratio (f/#) = focal length / aperture, and exit pupil = aperture / magnification. It also estimates the practical maximum useful magnification, roughly twice the aperture in millimetres.

Amateur astronomers and telescope shop staff use it to figure out which eyepieces suit a given telescope, avoid over-magnifying past the instrument's resolving power (which just produces a dim, blurry image), and match the exit pupil to their eye's dark-adapted pupil size for the brightest view. Enter the telescope's aperture and focal length plus an eyepiece focal length to see all four values.