Canon EF Extenders / Teleconverters
The range of Canon 1.4x and 2x lens extenders
The teleconverters (TC) will multiply the focal length of any EF lens used with them by either 1.4x or 2x.
So, a 300mm lens beomes a 420mm with a 1.4x TC and a 600mm lens with a 2x TC.
Sounds great, but it comes at the cost of reduced maximum aperture and slowing of AF performance (typ. 50% for 1.4x and 75% for 2x). The latest versions of the extenders are designed to reduce this, but there will always be some performance cost with an extender.
Maximum available aperture is reduced by 1 stop for 1.4x and 2 stops for 2x, so an EF 300/2.8 becomes 420/4 or 600/5.6
There are several versions of the converters in circulation, with the MKIII versions exhibiting the best optical characteristics, and lowest degradation of image quality (the extra glass in the lens unit will always have some effect)
Not all lenses can work with an extender. The list below shows suggested lenses that will work.
Note that apart from specialist lenses such as the tilt-shift TS-E range, teleconverters are designed for use with longer telephoto lenses. They also have the 3 extra electronic contacts for proper electronic communication with the extender. [See more details in the article about the EF lens mount]
EF Extender versions
1.4x EF Extender