The Canon Extender EF lenses are a group of teleconverter lenses made by Canon. These lenses are used between any compatible EF type lens and any of the Canon EOS line of cameras. When used with a compatible lens, they will multiply the focal length of the lens by a factor of either 1.4x or 2x, at the cost of decreasing the lens' aperture by 1 or 2 stops respectively. For example, using a 1.4x or 2x extender with the Canon EF 500mm f/4L IS USM would result in a 700mm f/5.6 or 1000mm f/8 lens.