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The East German antique lens, Zeiss Biotar 75mm, is famous for producing a swirling, water-like bokeh as illustrated in the photo below:

enter image description here

What optical materials and/or design cause this effect, which appears to be a coma aberration? Is it characteristic of any double Gauss design or is there something unique about the Biotar that creates it?

marked as duplicate by Michael C, Caleb, inkista, AJ Henderson Aug 24 '17 at 14:16

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