Teleconverters RF vs EF

Shawn1519 wrote:

Thinking, I just realized something about the RF Big White Lenses. Take for example a RF 600mm F4 lens - it is basically a EF 600 F4 mk iii with a built in RF to EF adapter. Putting a RF teleconverter on the RF 600 F4 gets you: camera -> RF tele -> RF to EF adapter -> 600mm Lens.

With my EF 500 F4 ii lens on my R5 it is: camera -> RF to EF adapter -> EF tele -> 500mm lens.

The RF 600/4 and EF 600/4 III lenses are optically identical, but the teleconverters are different. The EF teleconverter is designed to be 44 mm from the sensor (EOS DSLR, or EOS R System plus adapter); the RF teleconverter is designed to be 20 mm from the sensor. The result is just as you have described above.

Does switching the order of the adapter/teleconverter make a difference? I may have to borrow my wife's RF 1.4 tele and try: R5 -> RF tele -> RF to EF adapter -> Lens and see what the difference is (or is this combo even possible?).

In principle the adapter can take an RF teleconverter behind, or an EF teleconverter in front. Canon has made it so that the RF teleconverter doesn't physically fit into the adapter, but there is no optical reason why it shouldn't be done. (This would only be relevant if you own an EF lens and only have an RF teleconverter.)

Ironically, the very thing which Canon has prohibited by design, is what happens when you fit an RF teleconverter behind the built-in adapter of the RF 600/4.