This exercise captured differences in views of Nikon's M Plan 40X 0.65 n.a. 210/0 by 3 oculars,
namely Nikon's CFWN 10X-20, a generic Chinese WF10X 18mm high point
(which had been acquired to use with eyeglasses on an AO 120),
and another Chinese WF10X/22 acquired to use with eyeglasses instead of the CFWN 10X-20 on an Optiphot-66.
The Optiphot being used still suffers from slowly sinking stage, requiring frequent refocusing.
That stage was accidently bumped during ocular changes;
orientation of the calibration slide differs slightly among images.
Here is the WF10X 18mm peeking from a repurposed 1.25-inch telescope clamp
(which fits Nikon's trinocular T, but not F, head).
and an M42 focusing helicoid, which was adjusted to approximately equalize ocular-to-40mm lens spacing.
To my eye, the WF10X-18mm and WF10X/22 views are quite similar, except for field-of-view.
Afocal images captured using Canon 40mm pancake lens with a 6D instead of APC body
capture nearly full field from M Plan 40x 0.65 210/0
with fixed manual camera settings (f/2.4, 1/20 sec, ISO 4000)
The WF10X-22 is too large for the clamp, but the helicoid helped register the 40mm lens over it.
I had to hand-hold the camera for WF10X/22 and forgot to rotate the 6D from landscape orientation.
The 6D evidently found this WF10X-22 significantly brighter than the WF10X-18mm:
Visually, the CFWN10X-20 has lower contrast,
with calibration markings never appearing black when best focused.
Something obscures upper right edge for FoV > 18 in trinocular T left eye tube;
that happens neither in the right T eye tube nor either trinocular F eye tube,
which gets used for most imaging because of its empty photo tube.
apochronaut compared some higher magnification eyepieces:
Eyepiece anatomy (Olympus)
Could have paid $3 less @ AliExpress, but without rubber eye guards:
A pair of WF10X-18mm eyepieces were acquired in 2020 for an AO 120.
AO 180 oculars are OK, but lack relief for eyeglasses, which are not optional.
I was pleasantly surprised that corrections, if any, match so far as I can tell.
Nikon CFWN 10X/20 eyepieces are problematic for me, not only
lacking eyeglass relief, but also somehow provoking floaters to substantially obscure viewing.
Thankfully, WF10X/22 oculars, purchase June 2021 based on
seemingly match CFWN (lack of) corrections, work with eyeglasses mashed to them and mitigate floaters.
Curiously, CFWN 10X/20 oculars seem better in the AO 120, aggravating floaters less than in the Optiphot..??
The AO 120 is IMO a better diascopic microscope than the Optiphot,
but I am more interested in episcopic microscopy.
Another pair of high point oculars are wanted for the second Optiphot,
but benefits of 22 over 20 FoV do not for me justify their higher cost.