2x TeleConverter

A 2x TeleConverter or TC as it's more commonly known in its acronym is used to give you double reach in your lenses; i.e. a 50mm becomes 100mm but because you place another piece of glass in between your SLR's body and lense, it also looses by the same amount of light getting in...
There are a 1.4x converter...

So one normally uses a TC coupled with a fast lense, i.e. a f1.4 or in this case as below, I used my Tamron 28-75/f2.8 stop'ed down to f8 for a shaper focus...

This is shot WITH the TC at 75mm making it 150mm! Even though it's not recommended, by my, look at the cover of this jaded lampost! It sure has seen better days...

And this is shot without the TC and crop'ed in...
One would only use a TC when one has no choice. Really. And also, you need loads of light to get these babies to work! :)