Nikon D300 User Manual page 705

White Balance Tips and Tricks
When measuring WB with a gray or white card, keep in mind
that your camera does not need to focus on the card. In PRE
mode, it will not focus anyway, since it is only trying to read
light values, not take a picture. The important thing is to put
your lens close enough to the card to prevent it from seeing
anything other than the card. Three or four inches (about 75
mm to 100 mm) away from the card is about right for most
lenses.
Be careful that your lens does not cast a shadow onto the card
in a way that lets your camera see some of the shadow. This
will make the measurement less accurate. Also, be sure that
your source light does not make a glare on the card. This
problem is not as common since the card has a matte surface,
but it can still happen. You may want to hold the card at a
slight angle to the source light if the light is particularly bright
and might cause glare.
Finally, when the light is dim, use the white side of the card
since it has more reflectivity. This may prevent a No Gd
reading in low light. The gray card may be more accurate for
color balancing but might be a little dark for a good
measurement in dim light. If you are shooting in normal light,
the gray card is best for balancing. You might want to
experiment in normal light with your camera to see which you
prefer.
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