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Excerpt from Field Guide to Geometrical Optics
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, zPN=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'):