Nikon D810 - Manual (Page 332)

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This option controls how autofocus adjusts to sudden large
changes in the distance to the subject when AF-C (0 87) is
selected during viewfinder photography.
If Shutter/AF-ON is selected, both the shutter-release button
and the B button can be used to initiate autofocus. Select
AF-ON only to prevent the camera focusing when the shutter-
release button is pressed halfway.
a3: Focus Tracking with Lock-On
G
button ➜ A Custom Settings menu
Option Description
C
5 (Long) When the distance to the subject changes abruptly, the
camera waits for the specified period before adjusting
the distance to the subject. This prevents the camera
from refocusing when the subject is briefly obscured by
objects passing through the frame. Note that 2,
1 (Short), and Off are equivalent to 3 (Normal) when
3D-tracking or auto-area AF is selected for AF-area
mode.
(
4
D
3 (Normal)
)
2
E
1 (Short)
Off
The camera immediately adjusts focus when the
distance to the subject changes. Use when
photographing a series of subjects at varying distances
in quick succession.
a4: AF Activation
G
button ➜ A Custom Settings menu