Magnification specifies the ratio of an image dimension to its object dimension. Magnification is typically cited in one of three formats: axial magnification, radial magnification, or angular magnification. Axial magnification defines the ratio of an axial image shift to an axial object shift. Radial magnification defines the ratio of an image height to the object height. Axial magnification is proportional to the square of the radial magnification. Angular magnification defines the ratio of the image angle to the object height or angle. Several types of dimensions are employed in defining these magnifications: an axial dimension is a distance along the optical axis; a radial dimension is a distance across the optical axis, and may be linear or angular. A radial dimension may be called lateral or transverse, and an axial dimension may be called longitudinal. In a microscope, the "magnification" can be both radial and angular. The objective lens indicates a radial magnification for a real image, while an ocular lens specifies an angular magnification for a virtual image. A microscope does not typically specify an axial magnification, although it is an important parameter, since the depth of focus is scaled by the axial magnification.
The human eye comprises a static lens of the cornea and a dynamic lens within the ciliary muscle. The standard far point of human vision is infinity ∞, which may be expressed as zero diopters of magnification. A standard eye should focus on an object at 250 mm. This represents an addition of four diopters by the compressed dynamic lens. Therefore, the standard near point sNP of human vision is 250 mm, or four diopters of magnification. The process of dynamic focus within this range (250 mm−∞) is called accommodation.