The optical property of a lens or mirror system that corrects for spherical aberration, producing sharp images across the field of view.
From Greek 'a-' (without) + 'planos' (wandering, deviation) + '-ism' (condition). An optical engineering term developed in the 19th century as lens technology advanced.
Early telescopes had terrible aplanatism—you'd see a sharp image in the center but blurry chaos at the edges; the Abbe brothers solved this and accidentally created modern microscopy as a bonus.
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