Extender is Canon's name for a teleconverter. These high-performance accessories multiply the effective focal length of an attached lens by 1.4x or 2x. For example, a 200mm lens with Extender 1.4x has an effective focal length of 280mm: with the Extender 2x, this increases to 400mm. Extenders are only compatible with selected EF lenses, including the EF 70-200mm f/2.8L USM, EF 70-200mm f/2.8L IS USM, EF 70-200mm f/4L, EF 100-400mm f/4.5-5.6L USM, EF 400mm f/4 DO IS USM and fixed length L-series lenses of 135mm or higher.