Nikon D750 DSLR Camera (Body Only) - Manual (Page 158)

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AF-S focus mode : Focus locks automatically when the in-focus
indicator ( I ) appears, and remain locked until you remove
your finger from the shutter-release button. Focus can also
be locked by pressing the
A AE-L/AF-L button (see above).
3
Recompose the
photograph and shoot.
Focus will remain locked
between shots if you keep
the shutter-release button
pressed halfway (
AF-S ) or
keep the A AE-L/AF-L button
pressed, allowing several photographs in succession to be
taken at the same focus setting.
Do not change the distance between the camera and the
subject while focus lock is in effect. If the subject moves, focus
again at the new distance.
A See Also
See Custom setting c1 ( Shutter-release button AE-L , 0 336) for
information on using the shutter-release button to lock exposure,
Custom Setting f4 (
Assign AE-L/AF-L button , 0 361) for information
on choosing the role played by the A AE-L/AF-L button.