In article <4aap07$dj6@lectura.CS.Arizona.EDU>, bmtong@CS.Arizona.EDU says...
>Your choice will be determined mostly by one factor: the focal length >of the lens you will use. Obviously, if you put a 25mm extension tube >to a 24mm lens, you will go beyond 1:1 with ease, but the same >extension tube put onto a 300mm lens (maybe a xx-300 zoom) won't give >you any significant increase in magnification. For the 300mm, you can >only use a dioptre.

Hmmmm.... Actually, the choice between a diopter (use a two element achromat rather than a single element close up lens for better
results) and a tube depends on compatibility with a particular lens
(The short Nikkor teles, for instance, while great lenses for
normal use, are very poor on tubes [they work better with achromats],
while the 300mm f4.5 EDIF is both useful and excellent on tubes.)
BTW, a 25mm tube on a 24mm lens would place the focus point inside
the lens unless the lens were reversed.
Hope this helps.