At the moment, the widest-field ocular that I use on my 12.5" F/5
Dob is the 27mm Panoptic, yielding a 5.4mm exit pupil that is an
almost perfect match for my eyes, and is not wasteful in that sense.
However, I find it very frustrating to view the North America Nebula
through that scope/EP combination because the FOV is so restrictive;
in fact, in some ways, I prefer the view through my 7" F/5.4 scope
with the same ocular.

Now when I use that 40mm ocular on my 12.5" scope, my own pupil is
going to vignette the scope down to be the equivalent of an 8.5"
scope. Is this wasteful? Well, yes and no. The fact is, I don't
own an 8.5" scope. And although a 40mm widefield EP isn't cheap,
it's a whole lot cheaper than an 8.5" scope! I could take my
7" scope with me whenever I want to view the N.A. Nebula, but
since I've already got my 12.5" scope for other subjects during
the same viewing session, carrying my 40mm ocular is a whole
lot easier than carrying an extra telescope just for that one
subject.

- Tony Flanders

A nice real-world example - thanks. You know, the word *waste* is not the
main issue, but rather the frequent recommendations in this newsgroup that
folks not buy/use eyepieces that will result in over-sized exit pupils.
(Note that this advice is given, even though there is no hard rule on what
constitutes an *oversized* exit-pupil anyway!) I think that this notion is
falsely supported by talking about wasted light/aperture, because surely
waste is bad?! This is why I dislike the use of *waste* - because I think
it is misleading and confuses the issue, especially for the less experienced
observer.

Dennis