Nikon Camera N90 - Manual (Page 139)

About reflectance
When using the Center-Weighted or Spot Meter, always
keep in mind that the exposure indicated will assume that
the subject’s reflectance is equivalent to 18%, If the
subject varies from this reflectance, you must make an
adjustment to exposure. Generally speaking, a white
subject will have about a 90% reflectance, and an
adjustment of 2.5 f/stops (further open) will bring the
exposure back to the equivalent of an 18% reading. As
another rule of thumb, when shooting a landscape, the
light meter reading from green grass is roughly equivalent
to 18% reflectance.
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