And this is what TDP writes about using extenders on the 600/11
Is the Canon RF 600mm F11 IS STM Lens right for you? Learn all you need to know in The-Digital-Picture.com's review!
www.the-digital-picture.com
"The addition of a 1.4x Extender to the RF 600 creates an attractive 840mm IS lens. Extend the focal length without increasing the aperture opening, and the effective aperture is reduced — by 1-stop with the 1.4x mounted. Few are going to find f/16 attractive for most 840mm uses with diffraction impact showing even on lower-resolution R-series cameras. Magnifying the image and adding optics to the path are not helpful from an image quality perspective. That said, in bright light, the 840mm results from this lens can be usable.
The addition of a 2x Extender creates an impressive-sounding 1200mm focal length lens. In this case, the aperture is reduced 2-stops to a dauntingly-narrow f/22. Autofocusing that combination is a superpower of some R-series cameras, including the
Canon EOS R5 and
Canon EOS R6. Adding the 2x extender to the optical formula significantly degrades image quality — here is the
comparison. The RF 2x adds a small amount of barrel distortion to the image and magnifies (blurs?) the lateral CA. While this combination works, I recommend getting closer without the 2x mounted."