In article <5q8lgm$v9@nntp5.u.washington.edu>, romulan@u.washington.edu says...

>I would like to know the differences between cheap and expensive
>teleconverters.

Sometimes, just the price, as with lenses...;-)
(An excellent 2X [MF] converter is the inexpensive Vivitar Macro,
and it includes a focusing extension tube for closer than normal
focus. An equally good 2X converter is the Nikkor TC201...;-)
Converters designed specifically for big, fast, long teles are
consistently expensive, and offered, as far as I know, only by
the main-line big camera manufacturers.

>I hear about the number of elements, but what number is
>considered good? At least 7? What's the max and min possible? How does
>this affect picture quality? Exposure, sharpness?

The best number of elements is the appropriate number for good
performance. For short-lens type 2X's, generally seven elements;
for good 1.4X's and long-lens 2X's, 5 elements.

>Why does Canon have an A and B to use with different length lenses. What
>is physically different between them? Do the aftermarket lenses separate
>the tow or just use one model?

Short lens converters physically (but not optically) fit all lenses;
long lens converters have projecting front elements that strike the
rear elements of most short lenses, but fit inside long lenses with
recessed rear elements (and compensate for the depth of the optics
in the rear of most long teles).

>Can anyone recommend a good aftermarket brand for the FD mount? Is Komure
>good? My local store just said that you get what you pay for, and offered
>to sell me a 7-element Komura. Of course, it was the only one they had at
>the time.

Dunno - but for the FD mount, I would buy either Canon, or the Vivitar
mentioned above...

>I have a cheap one, and I'm wondering how my pictures will improve by
>investing in a higher quality one.

Probably some, but short-lens 2X's on even the best well-matched
lenses require stopping down the lens to at least f5.6 (f11 effective
aperture, pretty slow...) for good performance (f8-11 for better
performance) - with bad matches (common), the situation is worse.
You should see some edge improvement with the better converter, though.
Hope This Helps
(David Ruether - http://www.fcinet.com/ruether )