V1.02
Thom Hogan’s Complete Guide to the Nikon D300
Page 379
and is positioned at the bottom of the camera facing up
towards the secondary mirror. The primary mirror is partially
silvered in the middle, allowing a bit of light to get through to
the secondary mirror, which then diverts that light downward
into the autofocus sensors via yet another mirror (light is green
arrows in cutaway view of camera, below).
How the autofocus sensors work is a bit difficult to describe,
as both the physical implementation and the theoretical
methodology used get quite intricate and complex. The
simple explanation is that the light coming off the secondary
mirror goes through what is known as a “separator” lens just
above the actual autofocus sensors. This splits the light into
two distinct “images” and the line sensors underneath