Nikon D6 Reference Manual page 1110

•
During viewfinder photography, the focus points for which AF
illumination is available vary with focal length.
Flash unit
SB-5000
SB-910/
SB-900
SB-800/
SB-600/
SU-800
wireless
speedlight
commander
SB-700
•
Depending on the lens used and scene recorded, the in-focus indicator
( I ) may be displayed when the subject is not in focus, or the camera
may be unable to focus and the shutter release will be disabled.
1058 Compatible Flash Units
Examples of focus points for which AF-assist
illumination is available
Focal length 24 mm
Focal length 17 mm
Focal length 24 mm
Focal length 24 mm
Focal length 135 mm
Focal length 135 mm
Focal length 105 mm
Focal length 135 mm