Question about Canon EF 2x III teleconverter

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.

According to Canon support, no.

https://www.canon.de/pro....

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.

Non

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.