I went through this in 2020, comparing the RF 100-500mm with the EF 100-400 II, EF 400mm DO II and the Nikon 500mm f/5.6 PF. Basically, the 100-500 was just about as sharp as the Nikon prime, outresolved the DO II (and with the RF IIxTC outresolved the DO II with the EF 2x) and as a bare lens was a match for EF zoom at 560mm. So, the two EF and the PF were quickly sold. What I find surprising now is that on the R7 the RF 100-500mm is slightly sharper than the RF 200-800mm at 500mm but it is vice versa on the R5. By the way, the RF 200-800mm at 800mm f/9 is sharper than the 400mm DO II at 800mm f/8 with TC as I can see from looking at my old tests. Also, as I have written before, the R7 + 100-500 rivals the R5 + 200-800mm. But, when you add the RF 2xTC, the R5 pulls ahead.
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