Nikon D5100 Reference Manual page 20

11_ Measuring Preset White Balance
If no operations are performed while the displays are flashing, direct measurement mode
will end when the exposure meters turn off. The auto meter-off delay can be changed using
Custom Setting c2 (Auto off timers; _ 160). The default setting is eight seconds.
11_ Preset White Balance
The camera can store only one value for preset white balance at a time; the existing value will
be replaced when a new value is measured. Note that exposure is automatically increased
by 1 EV when measuring white balance; when shooting in mode M, adjust exposure so that
the exposure indicator shows _+0(_ 65).
[] Other Methods for Measuring Preset White Balance
To enter preset measurement mode (see above) after selecting preset white balance in the
information display (_ 81), press _) for a few seconds. If white balance has been assigned to
the Fn button (_ 165) and preset white balance selected by pressing the Fn button and
rotating the command dial, the camera will also enter preset measurement mode if the Fn
button is pressed for a few seconds.
[] Studio Settings
In studio settings, a standard gray panel can be used as a reference object when measuring
preset white balance.
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