Optical reference
Tele Vue 17mm Ethos: focal length, field and the magnification it gives
What the Tele Vue 17mm Ethos measures and the magnification and true field it produces once you drop it into a telescope of a given focal length.
Published specifications
An eyepiece has no magnification of its own. The Tele Vue 17mm Ethos gives 38x in a 650 mm scope, 71x in a 1200 mm scope, 120x in a 2032 mm scope, because magnification is the telescope's focal length divided by this eyepiece's 17 mm focal length.
Its 100 degree apparent field becomes the true field of view once divided by that magnification: wider in a short-focal-length scope, narrower in a long one. The exit pupil equals this 17 mm focal length divided by the telescope's focal ratio, so at f/5 the exit pupil is 3.4 mm and at f/10 it is 1.7 mm.
The 2 in barrel decides which focusers it fits: a 2 in barrel needs a 2 in focuser or diagonal and will not drop into a 1.25 in one.