May I ask you to share how you arrived at that number?
I would think a 500 mm f/5.6 out resolves a 600 mm f/11. On a 50 MP sensor, doesn't diffraction limit the 600 mm f/11 to effectively 24 MP? By the time you crop the 50 MP image from a 500 f/5.6 to 24 MP, you get a FoV equivalent to ~ 720 mm (500÷√(24÷50)) if I'm not mistaken.
If I am missing something, I would appreciate it if you could point it out.
System resolution is determined by a combination of the MTFs of the lens and the sensor (plus complicating factors like from the Baeyer grid) and there is not a sharp diffraction cut-off. f/5.6 to f/11 is in a transition region where both MTFs affect the overall resolution. At much greater f-numbers where diffraction is purely limiting, you can easily calculate resolution from the diameter of the entrance pupil, but it is difficult in this f-number region with current sensors. So, I analysed experimental measurements. In one of my geekier moments, I plotted the MTF50 measurements by ePhotozine and lenstip of the best lenses where there are not aberrations on the 5DSR against f-number. On going from f/5.6 to f/11, the consensus best MTF50 values dropped by 12 or18%, depending on who measured them, say an average of ~15%. So the resolution of a 600mm f/11 would be the equivalent of ~0.85 x600mm for an f/5.6 lens, ie ~ 510mm on an f/5.6.
The usual wiseguys will jump in and point out that resolution is determined by MTF10 or MTF20 and not MTF50, but changes in MTF10 and 20 parallel changes in MTF50 (as measured by optyczne, the parent site of lenstip).