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Quote "So my question is, "Instead of using a large lens in the primary doublet/triplet as the ED element and thus adding to the cost, why can't the designers use a smaller correction lens toward the end of the light path that would use about a 1/3 of the glass a front element would use?""
I read the question as , in the case of an achromat the second element ,(in the doublet) was being reduced to 1/3rd the diameter of the objective and placed further away from the single objective lens, not the achromat had a third corrector lens placed further down the optical path. Hence my comment.
Philip
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