Canon EF Extenders are the ideal accessories for photographers who wish to increase the power and versatility of their existing lenses. EF Extenders work by increasing the distance between the film plane (or CCD) and the rear element of the lens. The extra distance increases the effective focal length of the lens to bring distant subjects closer. Canon EF Extenders are specially designed for many of Canon's large aperture EF lenses and retain all metering and AF functions with most of these. Please contact your Canon dealer for a full list of compatible Canon EF lenses.