Optical reference
Celestron Omni 40mm Plossl: focal length, field and the magnification it gives
What the Celestron Omni 40mm Plossl 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 Celestron Omni 40mm Plossl gives 16x in a 650 mm scope, 30x in a 1200 mm scope, 51x in a 2032 mm scope, because magnification is the telescope's focal length divided by this eyepiece's 40 mm focal length.
Its 40 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 40 mm focal length divided by the telescope's focal ratio, so at f/5 the exit pupil is 8 mm and at f/10 it is 4 mm.
The 1.25 in barrel decides which focusers it fits: a 1.25 in barrel fits both dedicated 1.25 in focusers and the 1.25 in adapter of a 2 in focuser.