The Kenko 2x teleconverter (also known as a tele-extender) will multiply the focal length of the existing lens by 2x, and will cut back the amount of light by 2 f/stops. For example, using a 2x teleconverter with a 300mm lens will effectively increase the focal length to 600mm. A lens set to f/4 will now be f/8
Teleconverters are not recommended for wide angle lenses.
In the Box
Kenko 2X MC6 Teleplus Teleconverter for Pentax 6x7/67