Nikon Z 6II Reference Manual page 129

D
Subject Tracking
The camera may be unable to track subjects if they:
•
are similar in color or brightness to the background,
•
change visibly in size, color, or brightness,
•
are too large or too small,
•
are too dark or too bright,
•
move quickly, or
•
leave the frame or are obscured by other objects.
Focus
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