The Canon Extender 1.4× II has worked very well with the 70~200/4L IS on every EOS body on which I have used it, including FF bodies (but I have not actually used the 6D, although there is no reason to doubt compatibility). If you want to keep costs down, look for the original version, which is optically identical, and can be found s/h. The T CINE Speed Booster ULTRA 0.71x Adapter from Metabones allows a Canon EF-mount lens to be mounted onto a camera with a FUJIFILM X-mount. The adapter's optics maximize the connected lens's apparent angle of view by a factor of 0.71x while providing a one-stop increase for the camera over the lens's aperture range. More Details. The important factor governing the use of the Extender EF 1.4X III is actually the lens you are using with it, not the camera. Extenders will only work with certain fixed focal length L-SERIES lenses of 135mm or great, and these select zoom lenses: 1 EF 70-200mm f/2.8L USM 2 EF 70-200mm f/2.8L IS USM 3 EF 70-200mm f/2.8L IS USM II 4 EF 70-200mm Extending the effective focal length of select Sony E-mount telephoto lenses, the FE 1.4x Teleconverter increases the lens's magnification by 1.4x for a narrower field of view. The dedicated design maintains full communication between the lens and E-mount mirrorless cameras, including auto-exposure metering, autofocus, and Optical SteadyShot I wouldn't do it. It's already a slow lens and the tele-con will only make it slower f9 to f14 in the case of the 2x. Very slow starting apertures! Since I wouldn't do it, I don't know if they work or not personally. Canon tel-cons do not work on all Canon lenses. Make sure the Rf 100-500 is listed I traded in my Tamron 150-600 and bought a 100-400mm II. The Tamron at 600mm and f/8 on the 5DIII is as sharp at the centre as the 100-400 II at 560mm with a 1.4xTC. It is true that at f/6.3 the Tamron is a bit soft but at f/8 it is very sharp throughout its whole range and supersharp from 300-400mm. EF (full-frame) lenses have a red dot, EF-S (APS-C sensor) lenses have a white dot. The EF-S lenses extend further into the camera body so will NOT mount to an EF TC. I have no problems mounting the Sigma 1.4 and 2.0 TCs to my (EF mount) Sigma 70-200 2.8 for use on my 500D but the TCs are useless on my (EF-S) kit lenses and my 60mm macro. I have the Canon 100-400 version II, Canon 400 5.6L Canon RF 800mm F-11 and the Sigma 150-600mm Sport along with both version III Canon teleconverters. My preferred R5 setup would be my Sigma 150-600mm with the Canon 1.4x III converter giving me a 840mm F-9 setup. Bob 420-700mm f/8-10. 600-1,000mm f/11-14. Canon RF 70-200mm f/2.8 IS USM. Not compatible. Not compatible. Canon RF 28-70mm F2 L USM. Not compatible. Not compatible. While it’s obviously disappointing that the 70-200mm won’t work at all with the teleconverters, and the 100-500mm only half works, both the 600mm and 800mm primes are fully 0wxAf.