Nikon N1538 Reference Manual page 113

A
Predictive Focus Tracking
In AF-C mode or when continuous-servo autofocus is selected in AF-A
mode during viewfinder photography, the camera will initiate
predictive focus tracking if the subject moves toward the camera while
the shutter-release button is pressed halfway. This allows the camera
to track focus while attempting to predict where the subject will be
when the shutter is released.
D
Continuous-Servo Autofocus
When Focus is selected for Custom Setting a1 (AF-C priority
selection; 0 235) and the camera is in AF-C mode or continuous-servo
autofocus is selected in AF-A mode, the camera gives higher priority to
focus response (has a wider focus range) than in AF-S mode, and the
shutter may be released before the in-focus indicator (I) is displayed.
A
The AF-Assist Illuminator
If the subject is poorly lit, the AF-assist
illuminator will light automatically to assist
the autofocus operation when the shutter-
release button is pressed halfway (some
restrictions apply; 0 342). Note that the
illuminator may become hot when used
multiple times in quick succession and will
turn off automatically to protect the lamp
after a period of continuous use. Normal
function will resume after a brief pause.
AF-assist illuminator
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