Nikon The U Button

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The U Button
At settings other than K (Choose color temp.) and L (Preset
manual), the U button can be used to fine-tune white balance
on the amber (A)–blue (B) axis (0149; to fine-tune white balance
when L is selected, use the shooting menu as described on
page 148).
Six settings in both directions are available; each
increment is equivalent to about 5 mired (0150).
Press the U
button and rotate the sub-command dial until the desired value is
displayed in the control panel.
Rotating the sub-command dial to
the left increases the amount of amber (A).
Rotating the sub-
command dial to the right increases the amount of blue (B).
At
settings other than 0, an asterisk (“U”) appears in the control panel.
Control panel
U button Sub-command dial