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The locations of the six cardinal points of an existing lens may be determined by four means: the nodal slide, the foco-collimator (or swinging collimator), the two-magnifications method, and the use of the Newtonian lens formula. The cardinal points consist of three pairs of points located on the optical axis of a rotationally symmetric, focal, Optical system.  Cardinal points provide a way to analytically simplify a system with many components.