How does a 31mm Nagler work?

Question

Does it really fan out the light inside until it's like 4 inches wide and then compress it back down until it can fit in the pupil?

Cause that's what it looks like..

Answer 1

Hi. Pretty much. It uses a series of lenses to get a HUGE field of view at the cost of what is called "kidney effect", an apparent dark, kidney shaped spot that moves around as you move your eye. : http://www.astromart.com/articles/article.asp?article_id=276

Answer 2

, too, compared a Nagler 31 to a Leitz 30mm (older silver barrel with
blue engraving, same optics as all the others). I did the comparison
using my Traveler (f/5.8), observing star fields, Jupiter, and some
daytime observing.

I purchased my Leitz 30mm eyepiece over 4 years ago, well before this
eyepiece was widely known (I believe mine is the second one imported into
the US). It provided the widest, most comfortable field of view of any
eyepiece I had ever used (I called it my Nagler 30mm Plus). I was thrilled
to hear Al Nagler finally would introduce a similar eyepiece (since the
Leitz was so expensive and hard to find). I doubt the Nagler 31 would
ever have come into existence without the presence of the Leitz 30 (until
then, who would have thought people would pay $1000 for an eyepiece...).
Well, on with my comparison.

As others have pointed out, the Nagler has a slightly larger true field.
What immediately struck me about this eyepiece (aside from the appearance
and huge size) was how sharp the image was almost to the edge. Stars
maintained a small pinpoint character. In the Leitz, as others have noted,
the sharpness trailed below that of the Nagler about 50% of the way out.
Though, since I use the outer field for mainly aesthetic purposes (I don't
concentrate on the outer edge normally), the Leitz still functioned very
well. Nevertheless, the Nagler is significantly sharper at the edge and
this is a real plus (though, it is evident this great edge sharpness came