In article <4dmafn$gg7@solaris.cc.vt.edu>, dgaff@vt.edu says...

>I was wondering what the difference is between an extension tube and a
>teleconverter. I know that extension tubes are just hollow tubes that
>increase the focal length of the lens, whereas teleconverters use
>lenses to increase the focal length. So when do you use what?

An extension tube increases the distance between the optical
center of a lens and the film, providing closer focus in an
incremental step, unlike the variable-length tube called the
lens barrel, which provides a way to move the lens toward and
away from the film to focus it in its normal range. There is
some loss in light as the diaphram is moved further from the
film, and some lens designs perform poorly on extension tubes.

A teleconverter is an optical magnification device which
multiplies the lens image size by the factor given in its
name (such as, 1.4x, 1.6x, 1.7x, 2x, and 3x). There is some
loss of light using these, also (one stop with the 1.4x, 2
stops with the 2x, and a bit more than 3 stops with the 3x),
and, again, some lens designs do not perform well on them.

Converters maintain the original focusing range, but add
magnification. Tubes add no magnification, but shift the
lens focusing range closer to the subject (and therefore
prevent focus near infinity). Both are useful for close-up
work, but only the converters are useful for narrowing the
angle-of-view.

To throw something else into the pot: the close-up lens (which
fits on the front of the lens, instead of the rear, as do the
tubes and converters [except, of course, the lens front
converters which can both shorten and lengthen focal lengths
while maintaining infinity focus, but generally at the price
of considerable image quality loss]), which shortens the focal
length of the lens (since the barrel length is not shortened
to compensate, the close-up lens provides closer focus of the
subject). There is no loss of light with these, but, unless an achromatic version is used, performance is generally poor,
especially with the stronger types.

With all of these lens add-on devices, using smaller f-stops
can help cover a lot of optical ills and provide acceptable performance. With optimum combinations at optimum apertures,
performance can be very close to that of the lens alone.
Experimentation is the best way to find out what lens works
well with what device at what aperture for what purpose.
Hope This Helps