Originally Posted by Alexis Powell
Use of the 16/24/30x DS on the 50ED isn't discouraged. The optical design of the 24/30x MC and older WF eyepeice is different (it lacks the Barlow-like bit that protrudes into the scope), and though they work as a 16x on the 50ED, Nikon did not recommend it. I haven't though it through but my assumption is that perhaps there is some vignetting on the 50ED. Might not be obvious under most circumstances, and no physical harm will come from that combo, so I have done so in the past.
I've got all three 16/24/30x eyepieces: The first version (the old 24x/30x WF eyepiece), still with a rubber eyecup, the second (MC) version with the screw-up/screw down eyecup, and the DS. They all work nicely on the ED50 (and the other fieldscopes). There's some very slight vignetting of the first two on the ED50, but it's only just visible and not at all relevant for visual use.
Note that the first two versions can also be used on original fieldscope, the Fieldscope/Fieldscope ED, because they don't protrude into the scope.
In actual use I personally prefer the MC version on my fieldscopes, simply because the eyecup works really well and the whole eyepiece isn't as big as the DS. It's also a bit brighter and has a bit more contrast than the old 24x WF, probably due to more modern coatings. The DS ... Well, it's got nice optics, for sure, but I miss having a "real" eyecup. And it's the heaviest of the three versions.