<https://stargazerslounge.com/topic/355140-100-degrees-eyepieces-are-thay-worth-it/#comment-3867426>
<http://schema.org/text>
"You're conflating true field of view with apparent field of view.  A telescope's focal length determines it's maximum true field of view.  In general, refractors generally have shorter focal lengths than SCTs and especially Maks.  Newtonians tend to span all focal lengths.  A telescope's true field of view determines the maximum amount of sky visible through it in one view.  This has nothing to do with the apparent field of view of the eyepiece used to view that image.\n \n\n\n\tAn eyepiece has a focal length and an apparent field of view.  The focal length determines the magnification.  Just divide the telescope focal length by the eyepiece focal length to calculate the magnification for the combination of the two.  This determines the image scale.  The apparent field of view is how big the image circle appears to the eye, and thus how much of the image at a given magnification is visible in one view.  To a first approximation, divide the eyepiece apparent field of view by the magnification to determine the true field of view on the sky of the combined system.  Thus, a 40mm eyepiece with a 50 degree apparent field of view would show approximately the same true field of view as a 20mm eyepiece with a 100 degree apparent field of view.  The exit pupil would also be half as big in the latter creating a darker sky background.\n \n"
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