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The example picture, shot with a vintage triplet (Staeble Kata 45mm/f2.8, wide open, on APS-C, no further crop), shows a curious shape of coma-like cross pattern in the right- and leftmost sculptures - symmetrical, very unlike double-gauss coma/oblique spherical (which tends to be more abrasive than me), and it could be pictorially useful.

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Bokeh is also... a bit odd with the bright center in a trioplan-ish sharp outer circle (image cropped from landscape)...

What is the actual combination of aberrations at play here, and what construction details should one look for when looking for a triplet with this kind of character (but maybe better overall IQ than this sample...)?

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  • Just curious what portion of the image was the second crop from? Upper left? Mid-center right? Etc? – Michael C Dec 24 '19 at 0:47
  • Mid fifth horizontally, upper four fifths vertically. – rackandboneman Dec 24 '19 at 0:49
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What is the actual combination of aberrations at play here,

The "spinning top" bokeh near the edges appears to be tangential astigmatism. Because the bokeh balls don't change their "width" (i.e., the diameter of the spinning top doesn't substantially change) further from the image center, the sagittal focus is fairly constant. But because the bokeh balls are getting squished the further out, that indicates tangential astigmatism—the tangential focus plane is changing as you move away from the optical center. This is also supported by the fact that in the first image, the linear lights are smeared along radial lines, increasing with distance from the optical center.

Normally astigmatism is best characterized when observing point sources at sharpest focus; in these cases, the lens is not focused on any of the lights. Hence, it's the deformation of the bokeh along tangential fan lines that gives away the tangential astigmatism.

What distinguishes this lens's astigmatism from the run-of-the-mill squishing of bokeh balls due to minor amounts of Petzval curvature is the radial smearing of the center of the soap bubble bokeh balls. (Note that astigmatism and Petzval curvature are related aberrations).

and what construction details should one look for when looking for a triplet with this kind of character?

I'm not ignoring this part of your question. I honestly have no idea what elements of the lens construction leads this aberration.

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    There also appears to be at least one pane of glass (probably at least two) between camera and the tree lights, and that's probably adding some to the distortion as well... – twalberg Dec 24 '19 at 4:32
  • @twalberg possibly. But the spinning top / "donut on a stick" bokeh is pretty clear and distinct, even at the edges, so that's not appreciably affected by planes of glass in front of the lens. – scottbb Dec 24 '19 at 4:47
  • In the top right area (near the blown out floodlight), there is no plate glass in the way... – rackandboneman Dec 24 '19 at 11:59

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