The 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 180 mm lens will effectively increase the focal length to 360 mm. A lens set to f/4.5 will now be f/9.
Teleconverters are not recommended for wide angle lenses.
Multi-coated elements result in crisper images