Can someone tell me

what exactly do extenstion tubes do?
and where you can purchase them

Ran_photography wrote in post #3548498
what exactly do extenstion tubes do?
and where you can purchase them

They basically allow you to focus closer to the subject giving you higher magnification by moving the focus of the lens away from the sensor. They are just hollow tubes (with electrical contacts) that fit between the lens and the camera body. If you use them with a macro lens around 100mm focal length , then a full set will allow you to get to 2:1 magnification. With a non macro lens around 50-70mm focal length theyb will allow you to get to 1:1 magnification (std for macro lenses)
You can get them at any photographic supplier or on Ebay. Jessops do them in the Uk, Adorama do pro-optic brand ones in the US and Kenko do them worldwide.
One thing to beware of are cheap extension tube sets which have no electrical contacts making them very hard to use, and the Kenko tubes will not work with EF-s lenses without some surgery to the tubes. However Kenko do seem now to have released EF-S compatable ones recently which you can find on Ebay.

brian V.