What is it? The extension tubes allows you to add macro capability to your lens and increases magnification. The Kenko tubes comes with three metal tubes, 12mm, 21mm and 36mm. I have stacked all these tubes and attached it first to my 400d follow up with my 75-300mm.
Why chose Kenko tubes? They are widely used and cheaper than Canon and they can deliver the same performance like Canon.
I read the tubes works excellent with prime and macro lens but also works fine for the zoom and telephoto lens. I will wait for my nifty 50mm f/1.8 to arrive. I have seen nice shots from the 50mm with the extension tubes. BUT I'm definitely getting a proper macro lens, maybe Canon 100m f/2.8 Macro or the Sigma's 105mm 0r 150mm. I'm wondering what kind of shots I can get if I add the tubes to these macro lens.
The difference with macro lens and lens + extension tubes, using macro lens you can stand far away from the subject while using tubes you have to be closer. If you are using tubes make sure you wear an invisible cloak or you might scare the insects. Then the disadvantage of using tubes, you have to use manual focus by going close to the subject or changing the focal length on the lens. One more thing, it won't work on EF-S lens like the kit lens because it can't be attached to the tubes. Here are some test shots using the tubes.
3rd Test:Extension tube 12mm + 21mm + 75-300mm, focal length 75-80mm, distance 25 cm
Extension tube 12mm + 21mm + 36mm + 75-300mm, focal length 75-80mm, distance 15 cm