>>3909614>I can safely proclaim that swirly bokeh cant be explained by optical vignetting.You're correct. Mechanical vignetting (which is what I guess you mean by "optical") exacerbates the issue, but it'd would have to be really extreme to produce swirlyness of bokeh *on its own*.
And even in that case, it'd be much harsher shaped (as if you snipped the bokeh balls with scissors) than the smooth ovals we usually see.
What causes it, usually and primarily, is
1. field curvature (also called "Petzval aberration")
2. assymetric optical vignetting (more/less in the sagittal plane vs in the tangential)
These two combined (and maybe also helped by mechanical vignetting in some cases, for even stronger effect), force the circular bokehballs that grow from point sources of light, to appear elongated rather than circular.
Since they grow more in one direction than the other (due to different level of defocus due to field curvature, and due to different level of vignetting).
As a sanity check, I tried to find some graphs as evidence to this asymmetric vignetting in photographic lenses that are known for swirly bokeh - since most of the literature is for astro and microscopy.
And low and behold, a Tessar lens, a design characteristic for its swirly bokeh.
>picrelatedNotice the difference in the first row (periphery of lens), between the sagittal and tangential planes.
So it's a combination of all the above, usually you need 2 or more factors to get a decent, relatively smooth effect.
However you can get something kinda "similar" if you go bonkers on one factor.
The easiest to do (since you can't grind elements), is to cause extreme mechanical vignetting. Try using a hood for teles, on a wideangle (or normal) lens. Or make a cylinder of black cardboard and wrap it around your lens.
Once you can see blacked out corners clearly, you'll notice around the edges of the image you get lots of swirling.
