Caption: The field of view (FOV) of telescopes is just a circular region which is circular because all the optical devices along the beam path have circular symmetry.

The diameter of the FOV (which is also the FOV in another meaning of the term FOV) is usually tens of arcminutes for small telescopes like the Celestron C8 telescopes.

FOV (i.e., in the diameter sense) and telescope magnification are determined by the focal length of the eyepiece---the smaller the focal length, the smaller the FOV and the larger the telescope magnification.

Credit/Permission: © David Jeffery, 2015 / Own work.
Image link: Itself.
File: Alien images file: alien_field_of_view.html.