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. Bottom line and why I would prefer one of the 150-600mm super zooms , there is no substitute for having a lens with the actual focal length vs a tel-con. If you think of it this way, 50mm is one power. So, your 200mm lens is equivalent to a 4x power binocular. Adding the 1.4x tel-con and you now have abut a 5x binocular. I have a 60D and a Canon EF 1.4 Extender II, I was wondering if it would be compatible with the Canon EF 70-300mm f/4.0-5.6 IS USM Lens? As others have said, the short answer is no. You can buy a Kenko 1.4x which will fit, but you probably wouldn't be happy with the results. hcMpKa. 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. 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 This tele extender can be used with fixed focal length lenses 135mm and longer (except the 135mm f/2.8 Softfocus lens), and the EF 70-200 f/2.8L, 70-200 f/2.8L IS, 70-200 f/4.0L, and 100-400 f/4.5-5.6L IS zoom lenses. Superb optically, it preserves the image quality of the lens it's mounted to and multiplies its focal length 1.4x. Canon lens extenders were designed in the era of film, making them fully compatible with full-frame, APS-H and APS-C EOS digital cameras. The trade-off to the 40% extended range of the extender is For comparison, these Double-crested Cormorants were shot in 2020 with the 5D mark IV using the EF 100-400mm L and EF 1.4X near the same place as above. 280 feet, 1/1000th, f/11, ISO 1000, 560mm. Double-crested Cormorants - 5D4 with EF 1.4X Extender. Osprey, 200 feet, 1/1250th, f/10, ISO 800, 700mm. 4.1: 4.1: PowerShot G1 X Mark II ** See: Limitations with Canon Dual Pixel raw files in Camera Raw and Lightroom. Casio Camera. Raw image filename extension.