Thick Lenses

The simplified model found for thin lenses can be generalized for thick lenses. In this case the each focal point has its own lens plane associated with it. Its location along the optical axis is where the focal rays meet with their parallel rays. These two planes are known as the principal planes of the lens. The rays from a point source that preserve the angle of transmission through the lens cross the optical axis at two "nodal" points. So a thick lens has six cardinal points associated with it that includes two sets of focal, principal and nodal points.

For further reading on lenses and optical systems one can look at these references,

Jurgen R. Meyer-Arendt, Introduction to Classical and Modern Optics, 4th ed., 1995, p. 50

Miles V. Klein, Optics, 1970, p. 80

Grant R. Fowles, Introduction to Modern Optics, 2nd ed., 1975, p.297