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Excerpt from Field Guide to Geometrical Optics
An afocal system is formed by the combination of two focal systems. The rear focal point of the first system is coincident with the front focal point of the second system. Rays parallel to the axis in object space are conjugate to rays parallel to the axis in image space. Common afocal systems are telescopes, binoculars and beam expanders.
The transverse and longitudinal magnifications are constant. Equispaced planes map into equispaced planes. The relative axial spacing changes by the longitudinal magnification .
Because the magnification is constant, the cardinal points are not defined for an afocal system, and the Gaussian and Newtonian equations cannot be used to determine conjugate planes. However, any pair of conjugate planes coupled with can be used. A convenient pair is the front focal point of the first system FF1 and the rear focal point of the second system F'R2.