Or maybe the RF mount has the same mounting ring as an EF but RF lenses protrude well into the camera housing to get a lower flange distance (i.e. the mount is not at the end of the lens, but part way up the barrel to make up that flange difference). It could mean that the camera housing doesn't save size, but some RF lenses could be more like pancake lenses on the body so the mounted size is reduced (for lenses where that's possible i.e. wide angle, slow aperture). EF-S protrudes into the housing somewhat and the lenses are built so they won't mount on EF mount cameras - I wonder if they could do the same with RF but have a more pronounced protrusion into the body?
If this were the case, then weather sealing shouldn't be impacted, mount adapters wouldn't be needed, and the same size saving options could be possible. The only caveat here would be no using an adapter to mount mirrorless lenses from other systems to the Canon system (not that Canon would mind), unlike Nikon's mirrorless which now has the lowest flange distance and should be able to mount anything once adaptors are built.
But then the throat diameter starts to limit lens design. I don't think Canon wants to do that with short back focus lens designs.
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