Magnifying Power Calculator

About this calculator

This calculator uses M = 25/f, where f is the focal length of a converging lens in centimeters and 25 cm is the standard near point — the closest distance at which a typical eye can focus comfortably. The formula compares the angular size of an object seen through the lens, held at its focal point, to the angular size it would have if viewed unaided at the near point, giving the apparent magnification of a simple single-lens magnifier.

Opticians, jewelers and hobbyists use this relationship to rate magnifying glasses and loupes, since a shorter focal length lens always yields higher magnifying power. Physics students studying geometric optics rely on the same formula to connect a lens's focal length to its practical usefulness as a simple magnifier, distinct from the higher magnifications achieved by compound microscopes and telescopes.