Nikon D300 Complete Manual page 59

Like the Bayer filtration, the microlenses are part of the sensor
itself and manufactured at the same time as the sensor.
We don't want light hitting the photosite from oblique angles,
because it might not get into the photosite's photodiode (the
gray section in the above illustration). So the microlens is
designed to take light hitting at all angles and redirect it
straight down into the photosite well.
The smaller sensor size also comes into play here. Getting
light from the rear element of a wide angle lens to the far
corners of the full 35mm frame means that light can hit the
extreme corners of the frame at a very oblique angle with
older lens designs. Nikon has done three things to make light
hit more perpendicular to the sensor's photodiodes for the
D300:
DX frame size—The smaller frame size of the Nikon DX
•
DSLRs mean light doesn't have to get "bent" as much to
reach the far edges of the capture area.
Microlenses—As already described, these small lens-like
•
additions to the sensor capture light hitting the sensor
from angles and reorient it more towards perpendicular.
New lens designs—Starting as far back as the 17-35mm
•
f/2.8D Nikkor, Nikon appears to have begun designing
wide angle lenses with a modified rear element design.
Essentially, all Nikkor wide angle lenses are now designed
Thom Hogan's Complete Guide to the Nikon D300
V1.02
Page 59