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I cannot speak for every single brand and model of eyepiece, but I can state that *nearly* every modern eyepiece (including all brands mentioned on this thread) is completely corrected for inherent coma (which does not mean you will not see coma, merely that it will not come from the eyepiece), and the only commonly marketed coma-correcting eyepieces that were sold were the Praetorias, and they haven't been produced for decades.
Understand that any such eyepiece would be unusable in any scope except a newtonian, so no manufacturer is going to do that.
As for field curvature, that is a "horse of a different color", i.e. there are significant field curvatures to be found in many eyepieces. And it is entirely possible that that field curvature could be of the same sign as the telescope, and therefore additive, making a field curvature visible that might be, on its own for each, far less visible.
The Nagler T5 line was scaled. That means that eye relief was a set percentage of the focal length, and also making the eyepieces scale in size. Ergo, the 16mm has 16/31 the eye relief of the 31mm and represents about the smallest eyepiece possible in that series.
To create a line of shorter focal length Naglers required keeping eye relief long enough all the way down to a 2.5mm focal length,which meant a design with constant eye relief in all focal lengths was the easiest way to design a series. They are small to be usable in small scopes and to allow easy binoviewing.
Ironically, the 16mm T5 Nagler has less eye relief (by 2mm) than the T6 Series.
Current production Naglers are the:
31mm T5--long enough eye relief for glasses
22mm T4 Nagler--ditto
17mm T4 Nagler--ditto
16mm T5 Nagler--very short eye relief
13mm T6 Nagler--small form, medium short eye relief (not glasses friendly)
12mm T4 Nagler--longer eye relief like the 17mm
11mm, 9mm, 7mm, 5mm, 3.5mm, 2.5mm T6 Naglers--identical form and eye relief to the 13mm
The 26mm T5 and 20mm T5 are no longer in production.
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