Extension Tubes (Macro Photography)
Extension tubes look like rings and have no crystal inside of them. They are placed between the camera body and the lens. The farther the lens are from the camera's sensor, the closer you can focus. It's like using a loupe - a simple physics trick.
Extension tubes are meant to take macro pictures - you can put them on any lens. They are usually sold in sets of three: 12mm, 20mm and 36mm and those measurement represents how far the lens will be when you place the extension tube between your camera body and the lens.
There are some extension tubes that allows you to use autofocus because they are able to communicate electronic functions.
Here is a set of sample images taken with a 50mm lens on a full frame camera (so there is no crop factor).