Editing Magnification (section)

==Size ratio (optical magnification){{anchor|Optical magnification}}==
'''Optical magnification''' is the ratio between the apparent size of an object (or its size in an image) and its true size, and thus it is a [[dimensionless number]]. Optical magnification is sometimes referred to as "power" (for example "10× power"), although this can lead to confusion with [[optical power]].

===Linear or transverse magnification===
For [[real image]]s, such as images projected on a screen, ''size'' means a linear dimension (measured, for example, in millimeters or [[inch]]es).

===Angular magnification===
For [[optical instrument]]s with an [[eyepiece]], the linear dimension of the image seen in the eyepiece ([[virtual image]] at infinite distance) cannot be given, thus ''size'' means the angle subtended by the object at the focal point ([[angular size]]). Strictly speaking, one should take the [[tangent]] of that angle (in practice, this makes a difference only if the angle is larger than a few degrees). Thus, angular magnification is given by:

<math display="block">M_A=\frac{\tan \varepsilon}{\tan \varepsilon_0}\approx \frac{\varepsilon}{ \varepsilon_0}</math>

where <math display="inline">\varepsilon_0</math> is the angle subtended by the object at the front focal point of the objective and <math display="inline">\varepsilon</math> is the angle subtended by the image at the rear focal point of the eyepiece.

For example, the mean angular size of the [[Moon]]'s disk as viewed from Earth's surface is about 0.52°. Thus, through [[binoculars]] with 10× magnification, the Moon appears to subtend an angle of about 5.2°.

By convention, for [[magnifying glass]]es and optical [[microscope]]s, where the size of the object is a linear dimension and the apparent size is an angle, the magnification is the ratio between the apparent (angular) size as seen in the eyepiece and the angular size of the object when placed at the conventional closest distance of distinct vision: {{val|25|u=cm}} from the eye.

[[File:basic optic geometry.png|thumb|A [[thin lens]] where black dimensions are real, the greys are virtual.]]