This tube fits between the lens and the camera body and allows for closer focusing by extending the lens. The 34mm tube has a fairly large effect at bringing in the close focus range of a lens, but will have a noticeable drop in exposure value. Most commonly I will fit this tube to the 120mm macro as it needs lots of extension to get in close.
Tubes and extenders can be stacked to get more extension. 9mm, 17mm, 34mm, and 67mm tubes are offered plus a vario extension tube with 22-68mm range and the extension bellows which can extend from 67mm to 204mm.
Note: Only one left new in stock - original list price was $740!!!