Nikon D300 Complete Manual page 58

colored pattern called moiré.
Before we leave the sensor filtration topic, we need to discuss
how information gets to and from the sensor, since it can also
influence the image results.
I've mentioned the photodiode already, the part of the
photosite that is exposed to light and makes the conversion of
light photons to electrons. What I didn't mention is that
photodiodes are sensitive to the angle at which light hits
them. The general rule of thumb has been that light must hit
within 15° of perpendicular for it to be accurately captured by
the photodiode. Light hitting at more oblique angles produces
less photon-to-electron conversion, which falls into a general
category of sensor design issues called "shading." We
photographers see a similar effect as vignetting in the corners
of our images.
In order to "align" the light more properly with the sensor, a
microlens is placed over each photosite. The microlens has
the task of trying to make the light hit the photodiode at a
more perpendicular angle, thus reducing shading effects.
Thom Hogan's Complete Guide to the Nikon D300
Moiré shows up as added "detail"
not in the original, usually with a
color pattern to it. In this example
I've exaggerated the contrast and
color so that you can see wavy
patterns that weren't in the screen
being photographed (the original
screen is silver with a tight diagonal
weave in a regular pattern—those
curvy lines and color changes don't
appear in the screen's pattern). You
get moiré most often from things
like screen doors, tightly woven
fabrics, and any other object that
has a small, repeating, regular
pattern of detail.
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