I have now used this 75X DS for a few days,and I think is a very impressive eyepiece..What is really impressive,in fact, is the performance of the ED50 at this max power(for its aperture)..the little Nikon stays quite bright with the fixed power ,7mm, 40X DS..I think The eyepiece is Brighter than the Zoom at 40x (less elements?),,but even if not brighter,the eye relief and FOV are soooo much better,than the whole experience is better beyond comparison.The ED50 becomes a MACRO scope,with its minimum focussing distance ..I think my scope close focus is around 8 feet,,(no more and maybe less!)So at 40X brings a lot of detail looking at small,close-by things, like flowers,insects etc..cool really ,and I think I would use this eyepiece quite a lot for this type of Macro observation with my kids,since they always are more interested in things at hand reach distance than in flying birds .DOF at close range is Very thin ,but Is a sharp view and clear focus is easy to attain..The 40X DS also has much better edge performance than other Nikon Wide eyepieces,as You can imagine,For the basic design of these eyepieces seem to be the same ,using different barlows to add magnification in the various models,,Hence ,the FOV of this model is basically the main Sweet spot of this design,showing better Edge performance and less lateral color than other models..another point in favor of this eyepiece.
In the Kowa 823 the eyepiece grants 64X,and the performance of this combo is very good...probably as good as it gets.. ,or close enough, because I star tested the scope with this eyepiece,and got great results...I actually was more interested in trying high power in the Kowa than using this eyepiece in the ED50,but I can recommend this eyepiece for the ED50 as well..maybe not as the main eyepiece,but as an alternative to the Zoom ,to combine with a lower power one..It is more enjoyable that I expected..