Magnification:

The magnification, also termed as power and symbolized X, is the level to which a telescope makes objects look nearer. (Really, telescopes raise the examined sizes of distant objects, though they do not appear closer in terms of perception.) The magnification is a measure of the factor by which the apparent angular diameter of an object is raised. The 20X telescope makes the Moon, whose disk subtends around 0.5° of arc as examined with the unaided eye, appear 10° of arc in diameter. The 180X telescope makes a crater on the Moon with an angular diameter of only 1 minute of arc (1/60 of a degree) show 3° across.

The Magnification is computed in terms of the focal lengths of the objective and the eyepiece. When f_o is the effective focal length of the objective and f_e is the focal length of the eyepiece (in similar units as f_o), then the magnification factor m is specified by this formula:

m = f_o /f_e

For a specified eyepiece, since the efficient focal length of the objective rises, the magnification of the telescope also rises. For a given objective, since the effective focal length of the eyepiece rises, the magnification of the telescope reduces.