Nikon D300 User Manual page 645

How big is the 3mm spot? Well, the Spot meter barely
surrounds the little AF point rectangle in your Viewfinder. It
is rather small at .11 inches. When your D300(S) is in Spot
meter mode and you move the AF point to some small section
of your subject, you can rest assured that you're getting a true
spot reading.
In fact, you can use your Spot meter to determine an
approximate EV range of light values in the entire image.
You can do this by metering the lightest spot in the frame and
the darkest spot. If this value exceeds 4 or 5 stops difference,
you've got to decide which part of your subject is most
important to you and meter only for that part. Something is
going to blow out.
On an overcast day, you can usually get by with no
compensation since the range of light values is often within
the recording capability of the sensor. On a bright sunny day,
the range of light exceeds what your sensor can record by as
much as two times. This range can often be as large as 12
stops total, while your sensor can only record a maximum of
6 or 7 stops!
Don't let the numbers make you nervous. Just remember that
spot metering is often a trade-off. You trade the ability of the
camera's multiple "averaging" skills to generally get the
correct exposure throughout the frame for the highly specific
ability to ensure that a certain portion of an image is
"spot-on". The choice is yours, depending on the shooting
situation.
If you spot-meter the face of a person standing in the sun, the
shadows around that person may contain little or no data. The
shadows will often come out as solid black in the final image.
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