FujiFilm X-H1 Owner's Manual page 92

Quick Focus
To use autofocus to focus on the subject in the selected focus area, press
the button to which focus lock or AF-ON has been assigned (the size of the
focus area can be chosen with the rear command dial). In manual focus
mode, you can use this feature to quickly focus on a chosen subject using
either single or continuous AF according to the option chosen for G AF/
MF SETTING > INSTANT AF SETTING.
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