There are very few microscope eyepieces with apparent fields larger than 55 degrees. Most "UW" microscope oculars have apparent fields that fall in the 52-56-degree range, regardless of focal length. Their "ultra widefield" descriptions relate to their peers and counterparts within the realm of clinical and industrial microscope eyepieces, where orthoscopic/flat fields are of primary importance and fields wider than 60 degrees are of little-to-no importance. Obviously, they don't qualify as "ultrawide" in the realm of telescope/astronomic eyepieces. The distortion introduced by (and inherent to) ocular apparent fields beyond 60 degrees is, essentially, a negative (if not prohibitive) aspect in a standard microscope ocular.
As an aside: the widest-field microscope eyepieces I'm aware of are the Leica/Bausch & Lomb 15x Ultra W. F. (~16.7mm focal length) and Nikon's 20x/15 UW (~12.5mm focal length). The B&L utilized a 23mm barrel. Reichert, Cambridge, and at least one other company also sold the same model under their own labels. Its field number is 20, which suggests an apparent field of ~62 degrees. However, its optical design utilizes an negative field pair, so an afov estimation based only on field stop diameter does not apply well enough. Many users estimate the 15x/20's afov at 68-70 degrees.
Nikon's 20x/15 UW (30mm barrel) may be the only other microscope eyepiece with an afov similar (perhaps even slightly wider) to that of the Leica/B&L's apparent field. It is rare and expensive.
I have seen mention of a CWF 25x/13 eyepiece by Unitron, but I've never seen a photo of one and consider its existence to be unlikely. Perhaps it is a special-order item.
(Another side note for JG: I finally found a Periplan GF 25x but have not yet had the chance to use it under the night sky. I was extremely lucky and spent less than US $40 for it. )
Best wishes and kind regards.
Edited by MisterDan, 16 May 2021 - 10:32 AM.