Vertebrate eyes and cameras both use lenses in order to collect more
light than a pinhole. However, as a consequence, only parts of the 3 D
scene are in sharp focus on the camera image plane. Thus, if 
is the distance of the image plane from the lens and the object is
at a distance 
then we have
Since
is fixed it appears that there is a precise distance
which is in sharp focus. In practice, there is a range of
where the focus is reasonably sharp called the depth of field. In order to focus at different objects, the mammalian eye changes its shape and thereby focal length, while the camera moves in the Z direction or changes lenses.
The image plane is coated with photosensitive material: