Apparent Field vs. True Field

An eyepiece's apparent field of view is the angular diameter, expressed in degrees (°), of the circle of light that the eye sees. It is analogous to the screen of a television (not the picture seen through it). Eyepiece apparent fields range from narrow (25° - 30°) to extra-wide angle (80° or more). The true field (or real field) of view is the angle of sky seen through the eyepiece when it's attached to the telescope. The true field can be approximated using the formula:

True Field = Apparent Field ÷ Magnification

For example, suppose you have an 8" Schmidt-Cassegrain telescope with a 2000mm focal length, and a 20mm eyepiece with a 50° apparent field. The magnification would be 100 x (2000mm ÷ 20mm). The true field would be 50 ÷ 100, or 0.5° - about the same apparent diameter as the full Moon.