In optics, crown glass denotes a family of optical glass with relatively weak visible dispersion, conventionally corresponding to a relatively large Abbe number. It is a category of optical behavior rather than one unique chemical recipe.
A positive crown glass element paired with a negative flint glass element can form an achromatic doublet: their chromatic aberrations partly cancel while their net lens power remains positive. The cancellation requires selected powers and dispersions; merely placing two arbitrary types of optical glass together is insufficient.
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