Canon Extender EF 2X III for EF Mount L-Series Lens Click on Thumbnail to Enlarge The Extender EF 2X III from Canon is designed for select Canon EF mount L-series telephoto lenses.It increases the focal length of a lens by a factor of 2x while still delivering superb image quality. To ensure the best possible image quality in all shooting conditions, it combines redesigned optics, improved data communication between lens and camera and enhanced environmental protection.