@HD

Congrats! That is a seriously awesome lens. I have debated between the minor IQ loss compared to the II and the weight savings of the Mark-III/RF version before. But both are simply excellent and among the best 600mm available. And seems by your testing you are quite happy with it.

But yes, I agree with Kayaker. It is definitely better the RF version based on Bryan's crops. Which is strange to me since I thought that both are optically the same? Just a different mount.

Not sure, someone smarter than me might be able to chime in.

Just spending some time at a local park birding as we speak. Beautiful day here in the PNW, life is good.