ALocking Focus with the BButton
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 Bbutton is pressed, at which point focus will lock and remain locked until the Bbutton 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.
ASee 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 AAE-L/AF-Lbutton.