Serious Question --- Barlow Vignetting

From time to time on Astromart and elsewhere on the web I see eyepiece reviews that say thing like: "...and then I put my [Insert name of your favorite 60 degree+ AFOV eyepiece here] into my Klee barlow, and boy it really knocked my socks off. The [Insert name of your favorite 60 degree+ AFOV eyepiece here] plus barlow combination is great because it gives such a wide field at such high powers."

Wouldn't a barlow like the Klee, Celestron Ultima, Orion Ultrascopic, etc., vignette the light cone in many wider field eyepeices, and thereby reduce throughput, contrast and resolution? I am interested in any rules of thumb, formulae, etc., anyone can provide to help determine whether a given eyepiece-barlow combination is vignetted.

Thanks,

Jim

Most of these "shorty" Barlows together with a
long focal length wide-field eyepiece will lead to
vignetting at the field edge. The edge of the field
will appear darkened. The center of the field will
be fine -- no change in throughput, contrast, or
resolution at the center.

I think it will be difficult to predict the
vignetting without knowing details of the eyepiece
design. If you are looking for a rough
rule of thumb, I can offer this from experience...

Divide length of barlow by focal length of eyepiece.
If the answer is around 2.5 or less you will have
vignetting.

Vignetting is caused by the eyepiece, NOT the Barlow. These Barlows are all designed with sufficient clear apertures for all reasonable amateur scopes. Vignetting occurs because the Barlows introduce divergence, which some eyepieces handle with aplomb (including MOST negative-positive designs) and others cannot. A 50 degree Plossl, for example, can handle little divergence without vignetting, while a Radian, Pentax XL, or some such can usually handle quite a lot. I can tell you from experience that a 7mm T6 Nagler in a 2.8x Klee doesn't vignette at all.
--
Mike