What Is An Extender?
An extender is the same thing as a teleconverter, it’s an optical accessory that sits between the lens and the camera and multiplies the focal length. One fact of note is that Canon call theirs ‘extenders’, whilst Nikon and other manufacturers call theirs ‘teleconverters’. This results in a split in terminology that you will find in books and all over the internet!
Extenders can be a very cost-effective way to reach longer focal lengths that are typically useful for sports and wildlife photography. Prices of longs lenses increase almost exponentially, but an extender can get you great results from existing lenses, at a fraction of the price. There are caveats though, and if you’re not careful, you can get some bad results too! More information can be found by reading this in-depth article that is linked here. This explains how and why to use them, when not to use them and also provides some reference tables of focal lengths with different extenders. The most common extenders are 1.4x and 2x, but Nikon also make a 1.7x version.
A 300mm lens with a 1.4x teleconverter becomes a 420mm lens. A 70-200 mm zoom becomes a 140-400 zoom when used with a 2x teleconverter.