Nikon D800 - Manual (Page 180)

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4
Select direct measurement mode.
Release the U button briefly and then
press the button until the L icon in
the control panel starts to flash. A
flashing D will also appear in the
viewfinder. The displays will flash for
about six seconds.
5
Measure white balance.
Before the indicators stop flashing, frame the
reference object so that it fills the viewfinder and
press the shutter-release button all the way down. The camera
will measure a value for white balance and store it in the
preset selected in Step 3. No photograph will be recorded;
white balance can be measured accurately even when the
camera is not in focus.
A
Protected Presets
If the current preset is protected ( 0 161), 3 will flash in the control
panel and viewfinder if you attempt to measure a new value.
Control panel
Viewfinder