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.
Canon has released six models:1.4x
Canon Extender EF Wikipedia
The extenders are constructed with metal bayonets, and engineered plastic ends. There are no moving parts, other than the lens latch lock. Three generations of the extenders exist: the older 1.4× and 2×, the 1.4× II and 2× II, and the 1.4× III and 2.0× III. The II versions have weather-sealing and anti-reflective surfaces inside the extender body. The optical construction of the 1.4× versions are a of five-element design. Both 2× versions use seven elements, although the optical formula of the II version is different to improve optical performance. The III versions of the extenders have improved autofocus performance as well increased image quality over the II, and their front and rear elements are coated with fluorine anti-smear coating to be easier to clean.
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 by 50%. When used with the 2× extender, autofocus speed is reduced by 75%. 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. 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.
The following is a list of EF lenses that are compatible with the Extender EF lenses together with their resulting focal lengths, and apertures. Also listed is the autofocus and image stabilization functionality.
Canon EOS bodies that have a high density, high precision auto-focus sensor with 45 or more AF points are able to autofocus at maximum apertures of f/8.Canon EOS-1D X Mark II
Canon EOS-1D X (with firmware version 1.1.1)
Canon EOS 5D Mark III (with firmware version 1.2.1)
Canon EOS 5DS
Canon EOS 5DS R
Canon EOS 7D Mark II
Canon EOS 80D
Canon EOS 77D
Canon EOS 800D/Rebel T7i
Canon EOS-1D Mark IV
Canon EOS-1Ds Mark III
Canon EOS-1D Mark III
Canon EOS-1Ds Mark II
Canon EOS-1D Mark II
Canon EOS-1D Mark II N
Canon EOS 3