Apparent (afov) versus True Field of View (fov)
The apparent field of view is determined by the manufacturer and is the area of the sky that the eye sees. The true field is amount the sky actual seen through the eyepiece when it's attached to a telescope.

Commonly used equation:
fov in degrees = afov / Magnification
Example: fov = 65 degs / 61x = 1.06 degs
 

More Accurate Equation
requires field stop specification for eyepiece
Fov = field stop (mm) * 57.3 (deg/radian) / Scope Focal length (mm)
Example (19 mm Panoptic in 9.25” F/10 SCT):
Fov = 21.3 mm * 57.3 / 2350 = 0.52 degs

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