At 200mm f4.0, AI servo, centre single spot AF point, ISO 1600, IS on, AF case auto. (for example)
I was in my back garden doing the usual photographing boring crap (as I do with a new lens).
I focused on the gable end of a house roof about 100ft away and found that the focus was on some trees possibly 30 ft behind the confirmed AF point.
I tried again this time on a chimney post with only sky behind it. Nothing was in focus.
Tried something closer circa 60ft (a green watering can at the end of my garden) the planter about 2ft behind it was in focus.
Each time the single AF point was much smaller than the subject so the camera should not pick up the background.
I'm at a loss. I will try again but do not hold any hope of improvement.