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."
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 ").
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.
Since glass does not transmit the ultra-violet light, quartz is used, but such lenses can only be spherically corrected and not chromatically.
These notes occur chromatically (meaning in order of the scale).
