Kenko teleconverters are available in various magnifications and multiply the effective focal length of an attached lens by a factor. For example, a 200mm lens with a 1.4x teleconverter has an effective focal length of 280mm: with the 2x teleconverter, this increases to 400mm. Not all lenses will retain auto focus with a teleconverter attached and you will loose 1-2 stops of light depending on which teleconverter is being used.