Nikon D810 - Manual (Page 122)

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A Locking Focus with the B Button
During viewfinder photography, focus can be locked using the B
button in place of the shutter-release button (0 88). If AF-ON only is
selected for Custom Setting a4 (AF activation, 0 308), the camera will
not focus when the shutter-release button is pressed halfway; instead,
the camera will focus when the B button is pressed, at which point
focus will lock and remain locked until the B button is pressed
again. The shutter can be released at any time, although the in-focus
indicator (I) will not be displayed in the viewfinder. Note, however,
that if Focus is selected for Custom Setting a2 (AF-S priority
selection, 0 307) and single-point AF is selected for AF-area mode,
the shutter will not be released if the camera is unable to focus in
single-servo autofocus mode.
A See Also
See Custom Setting c1 (Shutter-release button AE-L, 0 319) for
information on using the shutter-release button to lock exposure,
Custom Setting f6 (Assign AE-L/AF-L button, 0 349) for information
on choosing the role played by A AE-L/AF-L button.