This is a good topic to cover after telephoto lens. What is a teleconverter? And what does it do?
Simply put a teleconverter extends, or magnifies the image. NOT ALL LENS support a teleconverter!
You can find these with varying amounts of magnifications; for example Canon has a 1.4x and a 2x version. Using my 2x on my 70-200mm lens makes it 140-400mm, on the downside you lose a few stops of light. So the same f2.8 on my 70-200 lens becomes f4 – less light gets into the camera.
You may also notices some degradation in the image when using a teleconvertor.
These are only minor disadvantages so it is still worth having one in your bag if you become serious with photography.
Thanks for reading, keep on shooting, and, as always I look forward to your continued support.