El Puffino,
Yes, with that "superior" eye-relief, I meant "having significantly more ER than its competitors". I had to go and check my notes and to my embarrassment, in my memory, I had over-estimated the ER of the 21x Kowa. It "only" has 20 mm ER, which is good, but not superior (Leica's 20x has the same ER). OTOH the Kowa 27x eyepiece does have superior ER to any other native eyepiece from the "big five", but at the cost of field-of-view.
Here are the ERs and fields-of-view of the fixed "low-power" eyepieces from five manufacturers:
Kowa 21x: 20 mm 54 m/1000m
27x 32 mm 35 m/1000m
32x: 20 mm 38 m/1000m
Leica 20x: 20 mm 54 m/1000m
32x: 19 mm 40 m/1000m
Nikon 30xDS: 19 mm 42 m/1000m
Swaro 20x: 17 mm 60 m/1000m
30x 20 mm 42 m/1000m
Zeiss 30x 18 mm 40 m/1000m
zoom 43-20m
As mentioned previously (by Henry and Kimmo) the ER-data seems to be somewhat variable according to sources, but these figures correspond to my experience with my non-scientific
"digiscoping-ER-standard": the Nikon CP4500.
The measured eye-reliefs of the zooms can be seen here:
http://www.tvwg.nl/testrapporten/telescoop/zeisstelescopen65tflen85tfl.htm
Unlike shown in the graph, the new Kowa zoom should behave like the Zeiss zoom (not tried myself).
What this data IMHO means in practice:
1) If you are mainly a birdwatcher and only occasionally need digiscoping or use digiscoping as a means to document some of your observations, you can forget almost all my musings about ER. They all are just fine (including zooms): choose the scope/eyepiece you like most and you can surely digiscope with it. In good conditions you can get fantastic results with any of them.
2) If you become seriously interested in digiscoping as a form of high-power nature photography, you may start to wish for a wider field-of-view. You may see that for good pictures you have to get close to the target. This is something that many may not realize until they have done it for a while. Higher power eyepieces are easily available from every manufacturer (Nikon even calls those fixed 50x and 75x "digiscoping" eyepieces), but a 20/21x can only be had from Kowa, Leica and Swaro - and the Swaro 20x suffers from vignetting with the CP4500.
3) Eye-relief of the eyepiece is THE feature that determines whether a camera works with the scope or not - and then 2-3mm may mean a lot. "Professional" digital cameras have often large, high-megapixel sensors and thus usually large and long lenses, which require more eye-relief from the eyepiece. If you want to use one of these cameras for digiscoping, a short-ER eyepiece causes vignetting and you may end up wasting megapixels or having too much power & narrow fields-of-view. Practically only Kowa 27x allows you to use cameras like Canon G6 for digiscoping.
I hope I was able to elaborate my point a little further and I apologize the previous misinformation about the Kowa 21x wide-angle.
Best regards,
Ilkka