No, this extender does not fit the zoom lens mentioned. It is designed for fixed focal lengths of 135mm and longer. I need it for my fast 200mm lens to get a focal length of 400mm.
No, according to https://fr.canon.ch/pro.... This is due to the microprocessor contained in this series (III). It has nothing to do with the casing, as some comments claim.
Autofocus always works in live view mode because a different detection model is used. Phase vs contrast is much slower. If you have a Canon 1dx series, the normal Af also works because the camera works in the fast focus field up to f8.
I only need the EF 2x III very rarely, as I take wildlife photos with an R5 camera. I only need the D5 Mk4 cameras for landscapes, and there this extender works wonderfully with my 16-35mm f2.8, 24-70mm f2.8, and 70-200mm f2.8. In my opinion, it's also about the type of camera, but I can't judge that. All my cameras are full-frame.
I only need the EF 2x III very rarely, as I take wildlife photos with an R5 camera. I only need the D5 Mk4 cameras for landscapes, and there this extender works wonderfully with my 16-35mm f2.8, 24-70mm f2.8, and 70-200mm f2.8. In my opinion, it's also about the type of camera, but I can't judge that. All my cameras are full-frame.
I don't have a 70-300, so cannot give a definitive answer. However, the EF 2x III does work with my 70-200 F4 L, so I would be very surprised if there were to be a problem. Moreover, I can state that on both a 70D and an EOS R (with adapter) the autofocus worked perfectly. Canon do make lens with don't work with the extender when the rear element extends too far backwards. I think these may be some of the EF-S lens. However, the documentation for the lens states that this is the case.
The extender fits all telephoto lenses and telephoto zoom lenses of the L series. BUT: The autofocus does not work with this extender. For this reason, I have exchanged it for the smaller 1.4 extender. I have had good experiences with it.