Field of View: Apparent & True

Field of view (FOV) can be a tricky concept to understand at first, because there are two types: apparent and true.  The field of view that is specified by an eyepiece manufacturer is considered the apparent field of view (AFOV), and is the angular diameter of the image that appears in the eyepiece. Eyepiece AFOV’s range from 30° – 120°, with larger AFOV eyepieces generally having higher price tags. True field of view (TFOV) is the actual angular size of the sky that the eyepiece and telescope combination is allowing you to see. Common TFOV’s range between one-tenth of a degree to two degrees. For that reason, TFOV is related to AFOV by the magnification. Also, higher magnifications result in smaller true fields of view. The AFOV for an eyepiece never changes, but the TFOV is dependent on the magnification (which is also the focal length of the telescope divided by the focal length of the eyepiece, as shown below).

To determine the magnification, we use the equation given here.

Magnification = ( ƒ_scope ) ÷ ( ƒ_ep )

Then for a given AFOV we can determine the TFOV,

TFOV = ( AFOV ) ÷ ( Magnification )

Example:

Using my telescopes specifications, and a 17mm eyepiece with a 68° AFOV:

Magnification = ( 650 mm ) ÷ ( 17 mm ) = 38x

AFOV = ( 68° ) ÷ ( 38 ) = 1.8º

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