Extenders, generally known as teleconverters, increase the effective focal length of lenses. Canon Extenders are available in two strengths, 1.4x and 2x. As the names suggest, the 1.4x Extender increases the focal length of your lens by a factor of 1.4, and the 2x by a factor of 2.

Canon EF Extenders are designed for use with a number of telephoto and zoom EF lenses. They do not work with EF-S or EF-M lenses. They can be used with a compatible EF lens on a Canon 球探体育比分_欧洲杯足球网乐￥在线直播 System camera with an EF-球探体育比分_欧洲杯足球网乐￥在线直播 Mount Adapter. If you're using RF lenses, you need Canon RF Extenders, which are designed to work with certain RF lenses – more about these shortly.

Not all Canon lenses accept Canon Extenders. This is because of their construction – extenders have a protruding front element that will not fit into the rear of many EF lenses and some RF lenses. Compatible lenses have a recessed rear element, which creates space for the front element of the Extender. (In the case of the RF 100-500mm F4.5-7.1 L IS USM, you need to zoom to at least 300mm in order to move the rear optical elements enough to fit an RF Extender. It is not physically possible to use the Extender at the lower end of the lens's zoom range.)

Extenders are a relatively cheap and convenient way of enhancing your telephoto capability. But using them to increase focal length comes at a cost ? reduced maximum aperture. The 1.4x Extender causes a decrease of one stop in the maximum aperture of the lens, while the 2x Extender causes a loss of two stops. This means you gain extra focal length at the expense of losing some light. If you attach a 1.4x Extender to an EF 70-200mm f/2.8L IS III USM lens, for example, it will become in effect a 98-280mm f/4 lens. If you use the same lens with a 2x Extender, it becomes a 140-400mm f/5.6 lens. In the same way, the EF 70-200mm f/4L IS II USM lens has a maximum aperture of f/4, but with a 1.4x Extender, the effective maximum aperture becomes f/5.6, while with a 2x Extender it becomes f/8.

With the reduction of the maximum aperture, a side effect of this is that there is the possibility of the loss of autofocus when using an Extender with DSLR cameras. On many DSLR cameras, autofocus stops when the maximum effective aperture drops below f/5.6, regardless of the amount of light available. However, newer EOS cameras such as the EOS 77D, EOS 90D and EOS 7D Mark II and professional-level cameras such as the 球探体育比分_欧洲杯足球网乐￥在线直播 and EOS-1D X Mark III allow autofocusing at maximum apertures of f/8.

All lenses in Canon's EF lens range have a maximum aperture of f/5.6 or wider, so all EF-mount cameras will autofocus with any autofocus lens. The f/8 autofocusing ability is needed only when you are using a lens and Extender combination that reduces the maximum aperture of the lens to f/8 or smaller. Some lens and Extender combinations mean that the maximum aperture of the lens is maintained at f/5.6 or wider and so autofocus remains unaffected.

The introduction of the 球探体育比分_欧洲杯足球网乐￥在线直播 System series, with the large RF lens mount, has shaken things up considerably. As mirrorless cameras, the 球探体育比分_欧洲杯足球网乐￥在线直播 System range use their full-frame imaging sensor for focusing. The 球探体育比分_欧洲杯足球网乐￥在线直播 and 球探体育比分_欧洲杯足球网乐￥在线直播P feature Dual Pixel CMOS AF, while the 球探体育比分_欧洲杯足球网乐￥在线直播5 and 球探体育比分_欧洲杯足球网乐￥在线直播6 have Dual Pixel CMOS AF II. This means they have on-chip phase-detection focusing. Also, the 球探体育比分_欧洲杯足球网乐￥在线直播 System cameras can focus at f/22 and are not limited to f/5.6 or f/8 for autofocusing, as demonstrated by the introduction of the Canon RF 600mm F11 IS STM, RF 800mm F11 IS STM and RF 100-500mm F4.5-7.1 L IS USM lenses. In fact, they can even focus automatically when these lenses are mounted with the Canon Extender RF 1.4x or Extender RF 2x.

The Canon Extender RF 1.4x and Canon Extender RF 2x were announced in February 2020 for use with the 球探体育比分_欧洲杯足球网乐￥在线直播 System range of full-frame mirrorless cameras. As usual, using the Extender RF 1.4x incurs a 1-stop reduction in the maximum aperture and the Extender RF 2x brings a 2-stop reduction, but the lenses can still focus automatically. They are compatible with the following lenses, extending their focal lengths as below:

In late 2010 Canon updated the EF Extender range to the Canon Extender EF 1.4x III and Canon Extender EF 2x III. These Extenders match the high image quality of the latest EF lenses and ensure the fastest and most accurate autofocus performance possible. Each Mark III Extender features an integrated microprocessor chip to provide complete communication between lens and camera and ensure that the focusing can be completed as quickly as possible.

Note that in order to ensure that the Extender-Lens combination is detected correctly, the Extender should be fitted to the lens first before attaching the whole combination to the camera.

The Canon EF Extenders Mark III are compatible with the following lenses, extending their focal lengths as below: