These anastigmatic lenses, which are manufactured up to X 40, are chromatically and spherically corrected, and for a middle diaphragm the errors of lateral pencils, distortion, astigmatism and coma are eliminated.
A system is said to be " chromatically under-corrected " when it shows the same kind of chromatic error as a thin positive lens, otherwise it is said to be " over-corrected."
Of flint glass must be chosen; the latter, although the weaker, corrects the other chromatically by its greater dispersive power.
On this account the lines D and G' are united for ordinary photographic objectives; the optical as well as the actinic image is chromatically inferior, but both lie in the same place; and consequently the best correction lies in F (this is known as the " actinic correction " or " freedom from chemical focus ").
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