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The field stop diameter is what determines the true field of view of an eyepiece in any given telescope. Eyepieces with 1.25” barrels are limited to 27mm maximum field stop diameters. An eyepiece with 27mm field stop diameter could be a 40mm Plossl with 43 deg apparent field, 32mm Plossl with 50 deg apparent field or 24mm Tele Vue Panoptic with 68 deg apparent field. Each will show the same true field but at different magnification for a given telescope.
Eyepieces with 2” barrels allow for 46mm maximum field stop diameters giving much larger maximum true fields for a given telescope compared to 1.25” eyepieces. Examples include 50mm Plossl and 41mm Tele Vue Panoptic.
There is no reason one would want to only own 2” eyepieces since focal lengths and apparent fields with field stop diameters under 27mm only require 1.25” barrels and almost all are manufactured such. Tele Vue has produced some dual barrel eyepieces but the 2” skirt is simply a convenience as the eyepiece is really only 1.25” (13, 11, 9mm original Nagler, 16, 12mm Type 2 Nagler, 22mm Panoptic, 12mm Type 4 Nagler).
Your favourite eyepiece need not be 2” but one that frames the object you are viewing nicely and this could very well be a 1.25” eyepiece.
If you only have 1.25” eyepieces then a quality 1.25” star diagonal which allows for full illumination of 27mm field stop eyepieces is all you need. Note though with a 1.25” diagonal you would be using the edge of the reflective surface which may not be to the same accuracy as the centre but if you buy a quality one the difference may not be noticeable as the mirror may be oversized to avoid any edge defects being perceived. A 2” diagonal would mean the edge would not come into play if using only 1.25” eyepieces.
One 2” low power and one 2” med power wide apparent field eyepiece is all you really need in 2”, something approx 30-40mm and approx 17-22mm.
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