= Achromatic doublet
An <achromatic doublet> is an <achromatic lens> with two <optical lens> elements. In the contact <thin lens> model, their <optical powers> add and their primary colour changes cancel according to <achromatic doublet power balance>. The conventional positive <achromatic doublet> uses a converging <crown glass> element and a weaker diverging <flint glass> element; these names describe a common choice, not a universal ordering of every modern <optical glass> composition.
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