2 inch eyepieces

[Tonys]

I have a Revelation Astro Superview 26mm eyepiece, which is my only 2 inch. It works okay for me and I've been looking at others in the range, but I'm surprised that the 20mm one, with an advertised afov of 68 degrees, is a 1.25 inch. The 26mm has an advertised afov of 70mm and I'm a bit puzzled why you need all that extra glass for a 6mm difference in focal length and 2mm afov. I wonder if there are any inherent optical advantages of 2 inchers that are purely a function of the size of the optics ie would a 2 inch eyepiece of identical design, focal length and afov have an advantage over a 1.25 inch? Somehow to me it seems there ought to be, given that in optics bigger is usually better (resolution etc)

[Mr Spock]

It's to do with the size of the field lens. 1.25" can't get wider than 68° at 24mm. If you want wider then the field lens has to increase in size and that's when you go to 2".

[Tonys]

That explains it clearly. Thanks.