In Fig. 1.7, the ray that passes from the center of the object, at the maximum aperture of the lens, is normally known as the marginal ray. It therefore passes through the edge of the aperture stop. Conventionally, this ray is in the y-z plane, usually called the meridian plane.
The chief ray is defined to be the ray from an off-axis point in the object passing through the center of the aperture stop; although there can be an infinite number of such rays, we can usually assume, at least for centered systems, that the chief ray is also restricted to the meridian plane.
Figure 1.7 Simple lens system and aperture stop.