Nikon D300 Complete Manual page 249

V1.02
The key word in item #2 is "differences." Sky, for example, is
usually very bright; near subjects we photograph tend to be
80
less bright . You can probably guess that if the upper left and
upper right areas metered are considerably brighter than the
lower left and lower right areas and are mostly blue, then the
camera is going to use #2 and #4 to determine you're taking a
picture of someone with sky in the background. If it sees a
flesh tone under the current AF sensor, consider that
assessment confirmed. In such a case, the sky usually isn't
considered as important to the exposure, so the camera
adjusts its exposure to match what it sees in the other areas.
Just remember that it's the difference in brightness between
areas that is a primary key to the matrix metering system, not
the actual values measured.
However, note that no meter can perfectly deal with any
situation that has a higher contrast range (large variation in
brightness; remember I call this exposure dynamic range) than
the camera's dynamic exposure (which, by the way, describes
about half of the daylight scenes you might shoot). In scenes
with a large exposure range either the bright portions of the
scene will have to be overexposed or the dark portions
underexposed.
One thing that catches many by surprise is that the D300's
matrix meter sometimes tries to preserve highlight detail over
shadow detail in high contrast situations, especially if you've
selected Auto Area Autofocus. That's because a highlight,
once overexposed, is unrecoverable on a digital camera (on
print film, you could often recover something that was as
much as three stops overexposed).
Whether the camera picks the right thing to expose properly
depends upon a number of things:
80 An early Kodak study showed that most outdoor scenes tend to form a bell curve in
overall exposure range, with something around 7.5 stops being the peak (160:1).
Thom Hogan's Complete Guide to the Nikon D300 Page 249