As mentioned, the extender won't physically fit the EF 70-300mm L f/4-5.6 IS USM II, so IMO if you are going to spring for the EF 100-400mm L f/4.5-5.6 IS USM II, I would go for the RF 100-500mm L f/4.5-7.1 IS USM. I agree with Trevor that the EF 70-300L II is a great lens. EXTENDER EF1.4x II. エクステンダーとは、カメラボディと撮影レンズの間に装着して焦点距離を延長するためのアダプターレンズである。. 装着レンズの高い光学性能を維持しながら“エクステンダーEF1.4×II”は1.4倍に、また“エクステンダーEF2×II”は2倍に Canon EF 1.4x Extender. £176.99. Click & Collect. Free postage. or Best Offer. Only 1 left. Canon Extender FD 1.4x A Teleconverter for Telephoto FD lenses. St no u16752. £94.99.
I was too lazy to find it in the Canon material, but this is from the RF 1.4x TC product page on B&H. Lens compatibility in the description: RF 100-500mm f/4.5-7.1L IS USM (can only be used within 300-500mm range) RF 600mm f/11 IS STM; RF 800mm f/11 IS STM

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.

hi Blake, i agree with most of your suggestions! my canon 1.4xIII extender and 600 f4.0II work quite good, it is the 2xIII + 600 II that turn pretty mushy! i don't have any issue with my rev. III extenders and 300 II. but like you have noted, i should send my extenders and 600 II to canon and have them calibrate the gear and hoping the issue go Wondering to get either a 1.4 or a 2X extender with the Rf 100-500 lens. Two concerns, one is a lot of wildlife photography is in lower lighting conditions and the 2X drops it to 11/14. Also, was surprised to see that the extender only works 300-500mm (not 1-299mm). Anyone have any experience with t 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
Note: Kenko TELEPLUS PRO300 DGX teleconverters are compatible with only proprietary lenses (Nikon and Canon) and Tokina lenses (except Tokina AT-X 70-200 F4 FX VCM-S and opera 50mm F1.4 FF Nikon mount). Other manufacturers' lens and camera models, or side-party adapters, are not supported.
Le multiplicateur EF 1,4x III est compatible avec la plupart des téléobjectifs et des télézooms de la gamme professionnelle phare de Canon : la série L. Il permet d'accroître la focale en réduisant l'ouverture de l'objectif d'un diaphragme seulement. 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 .
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    • canon extender ef 1.4 x ii compatibility