AFOVs

AFOVs, or apparent field of view, are a measure of how wide the field of view appears when looking through an eyepiece. It is the angular size of the field stop as seen by the eye, expressed in degrees. AFOV is primarily a property of the eyepiece design and is largely independent of the telescope's focal length, though the telescope’s magnification affects the true field of view that results.

The AFOV can be estimated from the eyepiece's field stop diameter (F) and its focal length (f) using the approximate formula AFOV ≈ (F / f) × (180/π) degrees, or more precisely AFOV = 2 arctan(F / (2f)). This means that larger field stops or shorter focal lengths yield larger apparent fields. In practice, different eyepiece designs deliver different AFOVs but may share similar field stops.

True field of view (TFOV) is related to AFOV by the magnification (M) provided by the instrument: TFOV ≈ AFOV / M. Magnification itself is the telescope’s focal length divided by the eyepiece’s focal length (M = F_objective / f). Thus, for a fixed eyepiece, increasing magnification narrows the true field of view, while keeping the same AFOV.

Common AFOV values seen in eyepieces range from about 50° for standard Plössl designs, through 60–68° for many wide-field designs, up to 82° for certain Nagler-type wide fields, and around 100° for ultra-wide designs such as Ethos. A larger AFOV provides a more immersive view and more sky at a glance, but can involve eye relief considerations and edge distortion if the eye is not positioned correctly.

See also: field of view, eyepiece design, magnification.