Thank you Nick, David, Peter and Leon, I appreciate your comments.
600mm at F11 gives an objective aperture of 54.45mm, so not bad for a 50mm finder sized lens eh.
With the EOS R, it is a very lightweight, well balanced and easily handled combination.
To give an idea of how this 600mm F11 lens performs on earthly subjects, here are a couple of 1st light photos from our back garden.
1/1000 sec, F11, ISO1250 & ISO6400. Rainbow Lorikeets approx. 7 to 8 metres away.
Cheers
Dennis