in optics, the size of an image relative to the size of the object creating it. Linear (sometimes called lateral or transverse) magnification refers to the ratio of image length to object length measured in planes that are perpendicular to the optical axis. A negative value of linear magnification denotes an inverted image. Longitudinal magnification denotes the factor by which an image increases in size, as measured along the optical axis. Angular magnification is equal to the ratio of the tangents of the angles subtended by an object and its image when measured from a given point in the instrument, as with magnifiers and binoculars.

There is no theoretical limit to the amount of magnification possible in an optical system, but practical magnification is limited by the system's resolving power—i.e., its ability to form distinguishable images of objects separated by small angular distances. A unit of magnification commonly used in microscopes and telescopes is the diameter, the magnification in diameters being equal to the number of times the linear dimensions of the object are increased.