The Canon EF 2x II Extender is a high-quality teleconverter that preserves the image quality of the lens it's mounted to and multiplies its focal length 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/2.8 will now be f/5.6. Teleconverters are best used with longer telephoto lenses and are not recommended for lenses wider than 50mm, e.g. 28-105mm (35mm format).