A birder needs some help with eyepieces.

One figure you forgot to mention was the diameter of the spotting
scope's front lens, or the f/ratio of the system. These, together
with the focal length, determine the eyepiece selections which are
likely to give the most pleasing images.

If the diameter is 60mm, then the scope is operating around f/9.
For most eyepieces this is a "slow" f/ number, and just about
any well made eyepiece will work well. You should be able to
get pleasing results form Erfles, Konigs, Plossls, Orthoscopics,
RKE's, Kellners, and just about anything that isn't a Ramsden
Huygens (sp?) or simple lens. Go for wide field at a lower cost.

However, if the diameter is closer to 80mm, then the scope is
operating around f/6.5 or so. This "faster" f/ number requires
a more precise design, which handles abberations and a steeper
light cone better. Plossls and Orthoscopics will certainly
provide crisp views. Erfles and Konigs might be acceptable.
But anything that isn't designed for f/5 and below will not
be desirable.

Check out University Optics for Konigs and Orthos. Plossls
are easy to find, and are affordable. Just be aware that
wide field EP's may have to deal with field curvature on a
short focus refractor.

Hope this helps,
Larry G.

=====================
Larry said just about everything...

I have 4 Pentax EP's in my Case
8.5 XFmm and 12 mm FX along with 10 & 20 mm XW's...

They all work very well in my f4.6 scope and in my f/10 scope..
..
MY 13 mm & 17 mm Orion Strratus eyepieces are a different story...

marginal ..well a little better then marginal in my 4.6 Dob.. but
perfectly good in my f/10 SCT.

But at less then 1/3 of the price of the WX's the Stratus's are hard
to beat ....IF your spotting scope is relatively slow...

Always that IF,......

My advice is grab her & the scope and drive to a dealer...and try
using a few different eyepieces in her scope... My dealer sure would
have ABSOLUTELY no problem with that... I do not think many would...

Then let her pick what she likes....then YOU write the check...

Bob G.

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Bob G. wrote:
[ ... ]

Check out University Optics for Konigs and Orthos. Plossls
are easy to find, and are affordable. Just be aware that
wide field EP's may have to deal with field curvature on a
short focus refractor.

Hope this helps,
Larry G.

Just one thing - I can imagine that for birding the field of view
is of great importance. I'm not a birder, so maybe I don't know
what I'm talking about, but my impression is that bird tend to
move around a bit faster than M42. The ones that don't do it are
usually not much fun to look at, being dead and all that. I think
wide FOV may come handy.

Orthos have really narrow FOV. Maybe not all orthos, dunno, and
maybe not that narrow, but this is an extra thing to consider when
choosing EP.

Regards,

- Alex