Canon Patent Application: Mirrorless Supertelephoto lenses with built in extender

In this patent application, Canon illustrates quite a few super telephotos that would be native to mirrorless.  The patent application deals with adding in extenders built into the lens, without increasing the size of the lens.

They have a distinct large rear element and a back focus distance that is too small for the EF mount, when you also consider external extenders.

If a space for inserting the magnification conversion optical group is provided between the aperture and the image plane, the total length of the original optical system becomes long. Further, if the magnification conversion optical group is inserted in a narrow space, various aberrations of the optical system may increase. The   present invention provides an optical system in which a magnification conversion optical group can be inserted and removed, but the total length is short and good optical performance can be obtained. 

Canon RF 400mm F4.0 with 1.4x extender

Focus distance         392.58  
F number                 4.12  
Half focal length (°)    3.15  
Image height            21.64  
Lens total length      280.33  
BF                      36.01

Canon RF 600mm F4.0 with 1.4x extender

Focal length           585.00  
F number                 4.12  
Half angle (°)           2.12  
Image height            21.64  
Total lens length      467.45  
BF                      55.50  

Japan Patent Application 2022-022771