A simple hollow tube, metal or plastic, connecting a lens and the camera body.
The end result of having such a tube is that the distance from the back of the lens to the film surface is longer, which means you can take photographs at shorter distances to the subject. Extension tubes are thus commonly used for macro photography and sometimes for closeup portraits. Bellows are similar but allow for the lens-camera distance to be adjusted. Tubes are obviously fixed lengths, but are smaller and more convenient.
The drawback is that less light hits the surface of the film, so you have to compensate for that. Some camera systems (eg: Canon) have electronic components in their extension tubes which let the camera adjust automatically.
cf. bellows, closeup filter, macro.
Entry last updated 2002-04-03. Term 437 of 1487.
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