Magnification -- from Eric Weisstein's World of Physics

Magnification is the increase (or decrease) in size of an image produced by an optical system compared to the true size. The most commonly considered form of magnification is linear magnification, although angular magnification is sometimes also encountered.

Let the height of the object by and the height of the image by , then the (linear) magnification is defined by

In the paraxial approximation,

Angular Magnification, Lens, Microscope