More on Matrices - 3D Math Primer for Graphics and Game Development - page 185

Perspective projection in 3D also projects onto a 2D plane. However,
the projectors are not parallel. In fact, they intersect at a point, known as
the center of projection. This is shown in Figure 6.4.
Because the center of projection is in front of the projection plane, the
projectors cross before striking the plane, and thus the image is inverted.
As we move an object farther away from the center of projection, its ortho-
graphic projection remains constant, but the perspective projection gets
smaller, as illustrated in Figure 6.5. The teapot on the right is further
from the projection plane, and the projection is (slightly) smaller than the
closer teapot. This is a very important visual cue known as perspective
foreshortening.

6.5.1 A Pinhole Camera

Perspective projection is important in graphics because it models the way
the human visual system works. Actually, the human visual system is more
complicated because we have two eyes, and for each eye, the projection
surface (our retina) is not flat; so let's look at the simpler example of a
pinhole camera. A pinhole camera is a box with a tiny hole on one end.
Rays of light enter the pinhole (thus converging at a point), and then strike
the opposite end of the box, which is the projection plane. This is shown
in Figure 6.6.

Figure 6.6. A pinhole camera.