Question about Canon EF 70-200mm f/4L IS II USM

The Canon EF 70-200mm f/4L IS II USM lens is generally compatible with the Canon Extender EF 2x III, as it has a recessed rear lens element structure that allows the extender to be used. However, when combined with certain camera models, such as the Canon R5 Mark II, there may be limitations in the autofocus function, especially in normal viewfinder mode. The autofocus performance may be reduced or only work reliably in Live View mode.

It is important that the extender is first attached to the lens before the combination is mounted on the camera to ensure correct detection. The Canon R5 Mark II supports autofocus at apertures up to f/8, which is relevant when combined with the 2x extender (which reduces the aperture by two stops). As the lens with extender has a maximum aperture of around f/8, the R5 Mark II should in principle allow autofocus, but there may be limitations in practice.

To summarise: The lens is mechanically and electronically compatible with the Canon Extender EF 2x III, and the Canon R5 Mark II can in principle utilise this combination, but autofocus performance may be limited depending on the situation.