EF1.4X III: EF2X III: EF 135mm f/2L USM: 189mm f/2.8: 270mm f/4: EF 180mm f/3.5L Macro USM: RF Extender 1.4x does not fit. 07-22-2023 04:56 PM. I just bought a RF extender 1.4x and 100-500mm lens for my Canon R5. I have owned 2x extenders in the past and never had an issue with mounting it or any lens. Just tried to mount the new extender, didn’t think much about it and just mounted as usual. Turns out it won’t mount properly and wrote: The Canon EF 70-300mm DO USM manual says it will not work with the EF 2X II extender. Will it work (particularly the IS) with the newer EF 2X III extender? thanks Jerry No, it will most likely not be fully compatible with that lens. Check the compatibility chart. (Max. dia. × length): EF1.4X III: 72.0 dia. × 27.2 mm / 2.8 × 1.1 inch EF2X III: 72.0 dia. × 52.7 mm / 2.8 × 2.1 inch Weight: EF1.4X III: approx. 225 g / 7.9 oz, EF2X III: approx. 325 g / 11.5 oz Case: EF1.4X III/EF2X III: LP811 * All specifications are based on Canon's tests and measurements. It will depend upon the lens (only compatible with certain fixed focal length lenses). See the specifications HERE. And also do look here: EOS 90D Supplimental Information (search that PDF for 2x). It will show all the compatible lenses along with what group the camera will clasify things (A, B, The Canon EF 100-400mm f/4.5-5.6L IS II USM Lens is compatible with the Canon EF 1.4x III and EF 2x III Extenders. As is often the case, the focal length range of the with-extenders combinations is quite impressive with relatively little increase in size and weight. Mount Adapter EF-EOS R question about compatibility with Extender EF 1.4x III in Camera Accessories 11-14-2023; Interesting EF 400 f2.8 IS II issue - EOS-1D X Mark III rendered unresponsive in EF & RF Lenses 11-12-2023; EF 100-400mm f/4.5-5.6L IS II USM Lens switches to MF mid shoot in EF & RF Lenses 11-10-2023 Camera/Extender Compatibility Chart EF 1.4 x III and EF 2 x III extenders are recommended for use with EOS Digital Cameras. Lenses that can be used with Extender EF1.4X III and EF2X III Usable lenses: Lens Focal Length & Maximum Aperture EF 1.4 X III EF 2 X III EF 135mm f/2L USM 189mm f/2.8 270mm f/4 EF 180mm f/3.5L Macro USM 252mm f/4.5 360mm Pw7oht.