The eye, like every compound optical system, has six important reference points (and planes)—the so-called cardinal points. These are the two principal focal points, designated as F1 and F2 ; the two principal points, P1 and P2, and the two nodal points, N1 and N2. Once these six points are located (in fact, even the first four suffice), calculations and constructions for image position, image magnification, etc., are easily made.
A scheme for remembering the relative distances between these cardinal points was previously published.1 This method utilizes the
well known benzene ring, on which the cardinal points are placed, as in A of the figure. Any two opposite sides which are parallel are equal. Thus if the distance between any two points is represented by the two corresponding letters, P1F1=N2F2; P2F2=N1