Like close up attachments, extension tubes
are not very expensive. They usually come in sets of three or four, each
tube having its own length. By combining the tubes in various ways, as
well as using them singly, several different extensions may be obtained.
Each extension works within a limited range of distances, just as with
close up lenses, but the scope of the entire set covers images from
about one tenth to full life size.
You have a choice between simple manual
tubes and the more convenient automatic ones. Since there is only a
slight difference in price, the automatic tubes are recommended because
they let you retain all the automatic exposure functions of your lens.
If your camera lacks through the lens metering, figuring the extra
exposure needed with a tube in place can be a chore. Instruction sheets
provided with sets of tubes sometimes give exposure increase factors
along with lens to subject distances and fields of view for each
combination. If not, experiment, bracket your exposures, and keep notes
for future references.