No time to head into the field today, but I did get a chance to try the RF 100-400mm with the extender. Once again, it was impressive. Although no critter friends have yet offered to model for me, I hope an opportunity will come soon.
Even though the relative reach of this setup is a bit shorter than what I like for bird chasing, it would make a fantastic travel option for general critter photography and close-ups because of its light weight, if you can imagine a 560mm AOV in an compact form factor weighing 858 grams. And unlike the RF 100-500mm, the zoom retracts fully while the extender is attached.
Sure, it’s f/11, but the R6 can produce clean images with an insane amount of detail well into four-digit ISO. Looks like the 600mm DO lens might be facing early retirement!