The 3x teleconverter also known as a tele-extender will multiply the focal length of the existing lens by 3x, and will cut back the amount of light by 3 f/stops. For example, using a 3x teleconverter with a 300 mm lens will effectively increase the focal length to 900 mm. A lens set to f/2.8 will now be f/8.
Teleconverters are not recommended for lenses wider than 50mm.
Multi-coated elements result in crisper images