Out from midday until late afternoon in bright sunshine with the EOS R and RF 600mm, I see that the lens captures with a smidgen better contrast without a filter than with. There was plenty of light to shoot at f/16 with the 1.4X extender attached, but heat shimmer was an obvious limiting factor.
It’s hard to tell whether the noticeably lower acuity with the teleconverter was due to pollution, diffraction, or a combination of both. But images were such that upsampling the lens alone would give nearly the same result. A cheap, third-party hood should make a difference, especially in harsh light.
Next task is to dive into the instructions to see if I can find a focus point smaller than the too-big square “point.” There were a few inexplicably misfocused shots today, chalked up to either user ignorance about which setting works best, or heat shimmer rising off the grass.
Or both. Still learning and grateful to be able to!