RF 600mm f/4 L IS USM

[neuroanatomist]

It wasn't a problem with the RF 2x, it appears to perform better than the EF 600mm III version with a 2x.

https://www.the-digital-picture.com/...mp=2&APIComp=2

The difference is that Bryan's comparison is using different cameras with (slightly) different resolutions and different OLPFs (the 5DsR has a 'self-canceling' AA filter, the R5 has an AA filter (not sure if it's the newer design used on the 1D X III that has four layers instead of two), and Canon's DPP sharpens up AA filtered images very well.

When I compared the EF 2xIII with the RF 2x, I found the former to be sharper when using the same lens on the same R3 camera, with identical processing – pretty much the most apples-to-apples comparison possible, for the given sensor resolution (which is significantly lower than the 45-50 MP of the 5DsR/R5).

[HDNitehawk]

The difference is that Bryan's comparison is using different cameras with (slightly) different resolutions and different OLPFs (the 5DsR has a 'self-canceling' AA filter, the R5 has an AA filter (not sure if it's the newer design used on the 1D X III that has four layers instead of two), and Canon's DPP sharpens up AA filtered images very well.

When I compared the EF 2xIII with the RF 2x, I found the former to be sharper when using the same lens on the same R3 camera, with identical processing – pretty much the most apples-to-apples comparison possible, for the given sensor resolution (which is significantly lower than the 45-50 MP of the 5DsR/R5).

Many variables at play. If I understand your testing you are using EF lenses.
It sounds like you have a fair comparison of an EF lens with both adapters.

The question I see raised, does the RF adapter play better with the RF lenses. I doubt it does, I think some of the EF 600mm III comparisons were because of a weak lens.

To your first point, if the EF 600mm III had been tested with a R5, and the RF 600 tested with a RF it might be a very comparable test.

Looking at Bryans test the EF 600mm II vs the RF 600mm with 2x the center is better on the EF and the Periphery on the RF.
https://www.the-digital-picture.com/...mp=2&APIComp=0
Then the difference between the RF 600mm and the EF 600mm II.
https://www.the-digital-picture.com/...mp=2&APIComp=0

Looking at all the charts it appears that there might be an issue with the EF 600mm III tested. If it is a soft lens the 2x would magnify the softness.

My RF extenders show up tomorrow, it might be interesting to see how the RF 600mm stacks up against the EF 500mm II at the same and equal framing distances.