This small device fits between the camera's prime lens and the camera body.  It doubles the length of the lens.  A 50mm lens becomes a 100mm.  A 200mm telephoto lens becomes a 400mm telephoto.  However in doing so, the device also doubles the f-stop of the lens.  So an f4 aperture becomes an f8 aperture.  The small ring on the teleconverter just acts as a calculator of the f-stop; it has no impact on the photo.

This teleconverter is in excellent condition and comes with a rear thread cap and a carrying case.