Focus and principal planes of focal optical systems (with focus)

By definition, the image focus F' of an optical system is the image of the infinite point on the axis. The beam issued out of this point is made of parallel rays to the axis.

These rays focalise in F' after crossing the system. The location of the intersection points of each incident ray with its corresponding image ray is, in paraxial approximation, a plane which shall be called image principal plane of the optical system.

This plane cuts the axis in H', is the focal image of the optical system. Distance . H' is the image principal point. We proceed in the same way as for the object focus F, the object principal plane, the object focal distance .

Following figure 35, any luminous ray issued from F cust the object principal plane in I and comes out parallel to the axis, it cuts the image principal plane in I'. An incident ray parallel to the axis groing through I, also goes through I' the converges in F'. These two rays cross each other in I in the object space then in I' in the image space I and I' are therefore conjugated.

The principal planes are conjugated with an associated transversal magnification equal to 1.

F, F', H et H' are the cardinal points of the optical system.