Nikon Z 9 Reference Manual page 128

A
Locking Focus with the AF-ON Button
•
The AF-ON button can be used for focus in place of the shutter-release
button.
•
When [AF-ON only] is selected for Custom Setting a6 [AF activation],
focus can be initiated only by pressing the AF-ON button; the camera will
not focus when the shutter-release button is pressed halfway. Focus will
remain locked after the AF-ON button is released. To end focus lock, press
the AF-ON button again.
Focus
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