Page "Magnification" Paragraph 8

* Angular magnification — For optical instruments with an eyepiece, the linear dimension of the image seen in the eyepiece ( virtual image in 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: