Nikon D3300 Reference Manual page 102

AF-S focus mode: Focus will lock automatically when the in-focus
indicator appears, and remain locked until you remove your
finger from the shutter-release button. Focus can also be
locked by pressing the A (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 (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
Autoexposure Lock
Pressing the A (L) button in Step 2 also locks exposure (0 116).
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