This can only be zero if E1 = E2 ("thin" system without spacing) or (C1)1 = 0 (lenses of zero power).
This theorem has important consequences for lens correctors in general. On account of it, Ross started with a 2-lens afocal corrector with nominally zero spacing and zero thickness. The theory of this case is strictly analogous to that given above for aspheric plates, except that the aberrations of the corrector are generated by the powers and bendings of the lenses which may or may not, at this stage, have aspheric surfaces. From Eqs. (4.3), (4.4), (4.5) and (3.213) we have
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