Magnification is measured by comparing the sizes of the image and the object (linear magnification), or the sizes of two images obtained in different conditions (visual magnification).

Linear magnification M 1 is given by the ratio between the size of the image (real or virtual) h i and that of the object h o :

M l = h i /h o

Magnification = 1 means that the image is the same size as the object.

Magnification greater than 1 means that the image is bigger than the object.

Magnification smaller than 1 means that the image is smaller than the object (it is still called magnification, even though it is actually a reduction!).

Images (both real and virtual) produced by optical instruments are often inverted. In that case, by convention, a minus sign (-) is placed before the magnification value, denoting negative magnification.

Visual magnification M v is determined by comparing the observations made with an instrument and with the naked eye. If h it is the size of the image on the retina when using, for example, a telescope and h in the size when using the naked eye, the visual magnification is given by:

M v = h it /h in