In optics, describing a lens system that has no focal point; producing parallel rays of light instead of converging them to a single point.
From prefix 'a-' (without) plus 'focal' (relating to focus). This is a technical term that emerged in 19th-century optical physics to describe specific lens configurations.
Afocal lenses in telescopes were a game-changer for astronomy—they let you view objects at infinity without eyestrain because your eye's natural focusing system didn't have to work overtime, letting astronomers observe for longer periods.
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