So, multiple elements, or more than one piece of glass, are required to be in an eyepiece. The overall collection of glasses must have a positive prescription so that the image the telescope is holding in the air can be focused by the observer. To keep the observer from seeing the poor images of the outer regions of the field of view, the eyepiece will have a field stop built into it. This field stop defines the field of view that the observer can see. The better the eyepiece performs in the outer regions, the larger the field stop will be.
Eyepieces come in three standard barrel diameters.
Unitron, Sears, and other Japanese telescopes with oculars of this size are pretty much non-existent now, but you run into them once in a while. The reason for using more than one size is that the eyepiece is sampling a field of view that might be large or small. The 31 mm Nagler is the largest field of view eyepiece in today s sizes. I predict that someday a new standard will be formed that will allow larger fields of view to be used. Three inch or 4 inch barrels may be on the large Dobsonian telescope 20 years from now.