Compatibility information

These Extender EF lenses can only be used with lenses that have a compatible mount. While all EF lenses have mount designs that can be used with EOS bodies, only lenses with accordant mounts can be used with the extender EF lenses. These lenses must have the clearance to accept the protruding lens on the extender, as well as 10 (3 extra) electronic contacts for proper electronic communication with the extender. These extra electronic contacts are used for sending the correct aperture information to the body. When a compatible lens is used with a 1.4× or 2× extender, it will cost the lens a one or two stop decline in aperture size, respectively. Autofocus speed of any used EF lens is also affected. When used with the 1.4× extender, autofocus speed is reduced to a half. When used with the 2× extender, autofocus speed is reduced to a quarter. Image stabilization may not work on lenses that are equipped with an extender, or on certain EOS bodies.

Though not specified by Canon as such, users have reported that the tilt-shift lenses in Canon's line-up can also be used with the extenders[3][4][5]. However, when used with an extender, the lens will not pass on the correct aperture to the camera. The focal length stored in the Exif data will also be incorrect.