I just purchased the z5 and am have difficulty setting up BBF.
After an extensive search the only BBF tutorials I could find pertain to the 6and7 models and am assuming it's the same process.
The steps I am using are:
A7 Activation is OFF
A1 and A2 are set as release
F2 AE button is selected
On my D750 the shutter button is completely disabled and will work after the BBF has been pressed.
I find on the z5 the shutter button can still be pushed 1/2 way down without actually focusing.
Is this they way the shutter is supposed to operate?