Aplanat
Historical document, translated for reference. It reflects medical knowledge of the 1920s–30s and is not medical advice.
Summary
The aplanat is a symmetrical photographic lens consisting of four lenses that eliminates distortion of straight lines at the edges. However, it suffers from astigmatism of oblique incidence, which causes the sharpness of the image to decrease rapidly toward the edges.
Encyclopedia entry (1928–1936)
APLANAT (from the Greek aplanes meaning unerring), a photographic lens of symmetrical construction, consists of two achro[matic lenses...]; the total number of lenses in an aplanat is 4 (see figure). Like all symmetrical double lenses, aplanats do not distort straight lines at the edges of images. Aplanats are not free from "astigmatism of oblique incidence," as a result of which the sharpness of the image, while good in the center, rapidly decreases toward the edges. The rear lens can be used as a simple landscape lens with twice the focal length.
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“Aplanat.” Soviet Medical Encyclopedia. English translation of Bolshaya Meditsinskaya Entsiklopediya, 1st ed. (Moscow, 1928–1936), ed. N. A. Semashko. https://sovietmedicalencyclopedia.pages.dev/article/aplanat/