Sorry to bump this older topic, but I picked up a pair of these for binoviewing, and wow are they are clear and sharp with super tight stars. I only tried monoviewing with them tonight as I haven't modified them yet. I sort of force-threaded them a tiny bit into the 1.25" adapter and squared them as best I could. Seeing was maybe 8/10 throughout the observing session.
I don't know if the lowest power setting is *truly* 21mm or not, but I was able to easily resolve E, and barely resolve F in Trapezium in my 12" dob, which is surprising for that magnification - 72x. E was maybe sort of there in my 21 Ethos, F was simply lost in C's bloat. Same situation for my 17 ES92 and 19 Pan. 18 Fujiyama showed E clearly, and a hint of F - mostly in the form of an elongated C. But the impression of the lowest power zoom level in the Nikon was "yep, that's E and that's F". I repeated this observation a few times to make sure it wasn't just due to a lucky bout of good seeing.
The nebula was littered with tiny stars that are only intermittently visible in other eyepieces around this focal length. The only eyepieces that matched the persistence of visibility of those tiny stars were my 25mm Zeiss E-PL, 18mm Fujiyama, and 11mm TV Plossl. While they were visible in my other eyepieces, it was only intermittently so, and they were just super obvious and seemingly more numerous in the Nikon. They were obvious enough that you couldn't miss them, so if you didn't know they were there before, this eyepiece is the one that would show you. Most of my other eyepieces showed them only because I knew to look for them.
The texture in the heart of the nebula was extremely well pronounced in the Nikon as well. As good as the 18 Fujiyama, which for whatever reason, really makes the texture in the nebula pop. I think the color cast of both this eyepiece and the Fuji lend themselves well to bringing out the texture in the nebula.
I turned to my favorite planetary - the Eskimo Nebula, and zoomed all the way in to what is apparently a 7mm equivalent. The nebula was super bright, to the point where I was like "nah that can't be right". I popped in my 7 Fuji and 7 Nagler and they looked muted in comparison. I didn't do enough careful observing to see if the eyepiece is indeed 7mm when zoomed in, so it could be that it was lower power and thus just a brighter exit pupil, but that little nebula popped right out at me and I could plainly see the central structure even in direct vision. Ordinarly this nebula does not reveal any structural detail in direct vision at anywhere near this magnification - I typically need to double the magnification to see it. The central star was especially crisp and bright.
Unfortunately the zoom isn't parfocal, and is quite stiff when it gets cold, but optically these are outstanding little eyepieces and eye relief isn't horrible.
Now I just need to apply the mod to get them to fit properly, and looking forward to more observing sessions with them!