i think its known as a teleconvertor. An extension tube is for macro shots.
It increases your focal length, but as other posters have noted in previous threads, note the loss in f-stops when you use a tele-convertor.
It's called a teleconvertor. And note that you cannot suka suka plonk a teleconvertor behind any old lens. Since teleconvertors cause a loss in image quality and increased light fall off, it's highly recommended that you use teleconvertors only on telephoto primes most of the time, although good image quality can be gotten from using teleconvertors with the pro level-telephoto zooms (e.g 70-200mm f2.8)
Unless you want to get a decent picture, it's not recommended that you try with your 28-105 or 70-300mm consumer zooms. Normal min aperture at the long end is around 5.6 Add on a teleconvertor, you will be barely able to see any image at all in the viewfinder.
To get the best results, it's always better to get a real telephoto lens.
You are thinking of a teleconverter which is a device for extending the focal length of a lens by using further lenses in a small tube assembly. The main advantage of a Teleconverter or TC for short is that you keep infinity focus.
Extention tubes on the other hand have no optical elements and increase the image size. However you loose infinity focus. Extension tubes are used mostly for macro work and also with long lenses to reduce the minimum focusing distance, something that is extremely useful in for example bird photography.
Thats true for the shorter teles, but however, for instance bird photography soemetimes require 500/600mm lens. The price of getting a 600mm tele prime will be much greater than getting a 300mm lens + a 2x TC.
