Thanks everyone
I've been puzzling over the questions raised around the sky and darkening on the cactus.
Here's a color shot as ref:
We did have mackerel skies Pete, but I can't tell if this accounts for all the cloud pattern, or if the Fuji has added it's own to the mix. I'll check other frames again and see.
The darkening on the cactus is odd, because I saw it in PP before adding any FX, other than a red filter - but when I look at the cactus in color I can now see that it's darker at the top, so maybe the red filter brings this out in B&W PP?
It's still somewhat of a mystery as to how the Fuji does AF during a Pan - clearly there are pans where the subject is the landscape and it's all miles away, so it's not issue.
Then you have shots like this where the subject is in the foreground - so how does it accomplish AF when you get to it during the pan, without messing with the DOF? I suspect it sets focal distance at frame 1 and then keeps it there - anyone know?