I've been doing some backyard testing, and I think I'm starting to see an interesting pattern.
I'm shooting with my R5 + the RF 800 f11, which has a relatively long MFD of 19.5 ft. But I also have a 35mm extension tube which shortens the MFD to 14.5 ft. My test subjects are the numerous Gold Finches we have in the garden area, near a feeder. Now, when I have the choice of how close I want to get to my subjects, I've always tried to be like MAX close, but still be able to lock focus. So with my 800 F11, that would be like 19.6 ft I literally focus on a perch, try to focus, and get a red box. Back up an inch, red box, one more inch, blue box. Okay, I'm good Same thing when using the extension tube, except now I end up at like 14.6 ft.
But doing this ^ I think I'm starting to see a pattern. In a nutshell, its starting to feel to me that while my 800 F11 "can potentially get a sharp focus" at 19.5 ft... or 14.5 ft with the extension tube. It just seems to miss a lot more ? I don't know anything about what I'm about to say, but it just seems like the auto focus system is a lot happier, if it can focus back and forth, a little beyond, and a little closer than the target, then find the sharp spot.
I've done all this same kind of stuff with my Sigma 150-600, and it seems to focus quite a bit better at about 13 or 14 ft, than it does at its listed MFD of 11.5 feet also.
Just a guess, but Id say "the optics" of these lenses are, in fact, capable of a tack sharp focus at the listed MFD. But the (eye) auto focus system of my R5 seems to prefer a little leeway both directions, to be able to nail critical focus more often.
Am I just completely full of it ? Or is this something any of you are aware of, and agree with ?