Optipedia • SPIE Press books opened for your reference.

Nodal Points

Two additional cardinal points are the front and rear nodal points (N and N′) that define the location of unit angular magnification for a focal system. A ray passing through one nodal point of a system is mapped to a ray passing through the other nodal point having the same angle with respect to the optical axis.

Both nodal points of a single refractive or reflective surface are located at the center of curvature of the surface:

The angular subtense of an image as seen from the rear nodal point equals the angular subtense of the object as seen from the front nodal point.

If n=n, z_PN=z'_PN=0, and the nodal points are coincident with the respective principal planes. The magnification relationship now holds for the Gaussian object and imagedistances (z and z' are measured relative to P and P'):