In article <41vk6a$1kc@news2100.microsoft.com>, danwel@microsoft.com says...
>Anyone out there have an opinion on teleconverters? (Is this a trick
>question?) Do they sharply degrade image quality, or are they a bona
>fide alternative to a genuine extreme focal length lens?

Bob Atkins wrote a very carefully written and thorough article
in the current (July-August) issue of NATURE PHOTOGRAPHER
magazine, which I recommend you check out. The short answer to
your question is that they can be both terrible and excellent,
depending on the quality of the converter, the matching with
a particular lens, the quality of the lens. There is no reliable
predictor of the quality of images from a combination - it must
be tried to see if it works for you (a $500 converter on a $1000
lens does not guarantee excellent [though maybe acceptable]
results, and a cheap converter on a cheap lens may be acceptable
[though probably not excellent] results. (See "SUBJECTIVE Lens
Evaluations [Mostly Nikkors]" in rec.photo.advanced [the
teles and converters listings] for examples of combinations that
do and don't work in the Nikon line. I forgot to add to the list
the Vivitar Macro Converter, which is an excellent inexpensive
2X with a focus helical added.)
Hope this helps.