[Brightcolours]

Yes, Dave, I would have to try before I buy!


As an aside, I believe that the 20MP (full frame) camera is not diffraction limited until about f/13, so for that camera, if I am right, it would not be absurd to expect sharpness.  And I was serious that the aperture stopping down can effect sharpness.  

Now about my fantasy...I have used the 70D, and you have been using the D500, both of which are APS-C, though I had to deal with a tougher 1.6x versus your 1.5x.  I used my 70D for Macro quite a bit at f/11 and didn't seem any the worse for it.  I know in theory there should be diffraction, but since it was often necessary for a sufficient DOF I had to use it.  Therefore I am not expecting it to be unusable with a telephoto.

Still, I would, in reality, want to go back to 20mp.  I can always do some shooting at f/11 with the 90D, but now that I think of it...I have been mostly shooting wide open.  If you can call F/5.6 wide, that is!

The truth that is sinking in, is that I am hooked.  I think I might actually want an RF camera!  Maybe not now.  But soon!

-Mac

Clearly diffraction limited past f4 (center sharpness drops). On 21mp.
But as Klaus has pointed out, 600mm will often be atmosphere limited at the usual long distance usage, limiting the impact of diffraction softening (or rather, masking it).