An extension tube is basically a hollow tube that goes between the lens and the camera body. By "extending" the lens further away from the sensor, it increases how close you can focus and the total magnification possible. Unlike teleconverters or screw-on diopters, however, they don't contain any glass elements so they don't degrade image quality at all.

You can by them separately (i.e. a 25mm extension tube) or as a set of 3 (usually something like a 12mm, 20mm, and a 36mm tube). You can use multiple tubes together so all three will give you 12 plus 20 plus 36 = 68 mm of total extension or any combination of the three. Single tubes run about $50, but you can usually get the set of three for around $100. It is a cheap way to get high magnifications, as long as you can get close enough to the animal.

How much extension you use determines the magnification. You calculate magnification by using

total magnification = total extension / focal length

Your Sigma 105 already goes down to 1:1 magnification, so it must effectively have 105mm of extension built in. If you added another 73mm of extension (the set I described above), the total magnification would be = (105 plus 68)/105 = 1.65x.

Because of the way extension works, shorter macros get more increase in magnification. The same 73mm of tubes on a 50mm macro lens would give you (50 plus 73)/50 = 2.36x magnification.

Most modern extension tubes are "auto" extension tubes which are specific for your camera mount. They can communicate between the lens and the camera, allowing autofocus, autoexposure, etc., to work. Kenko make a decent set for most camera brands.

The only down side to extension tubes is that you lose the ability to focus to infinity when you have them on.

This shot (Black Milksnake) was taken with a 90mm macro and 68mm of extension -



and the same three tubes on a 50mm macro lens -


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Chris Harrison
San Antonio, Texas

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