Hi Oscar,
As Mike says, best to switch to manual. But there is a half-way house setting available - Digital Manual Focus.
With DMF a half press of the shutter uses autofocus then you can fine tune the focus manually using the focus ring. I have only taken a couple of shots using this (See attached Grey Heron where autofocus was on the twigs then I tried to get the eye and bill in focus manually. It was a fair distance away, on a very overcast day and beyond the 600mm optical zoom range (into 2x ClearView option so equiv. of 1200mm), none of which exactly help.
As an aside, attached are a couple of other images taken in the 'ClearView' range in brighter weather. Wren at c1100mm and Waxwing at 1200mm (no crops or sharpening just resized for BirdForum purposes).
Cheers, John