I've seen lenses with round apertures that had horrible bright edges on out-of-focus highlights. And lenses with pentagonal apertures that provide a lovely smooth rendering. And tessars that were at both extremes.
One thing that I believe is true: smooth rendering of out-of-focus details behind the focus plane require slightly undercorrected spherical aberration. The spherical aberration is what fades the edges of the disks that are points when in focus. Most older designs did not fully correct primary aberrations such as spherical aberration when used at wide apertures, so that explains why they show a rendering that to me has a vintage look. But most become quite sharp when stopped down. The trick for a lens with good bokeh is that as one stops down, the sharpening up of those disks (due to reducing the spherical aberration) is matched by those disks becoming smaller (due to increased depth of field). Those are the lenses that seem to render beautifully at all apertures. I have an old Ilex Paragon (an inexpensive but good coated tessar design from the 50's and 60's) that comes closest to that ideal among my large-format lenses. But I usually am trying to make everything sharp, or I'm not that concerned about the rendering of unsharp bits, when using large format. When it matters, that's the lens I usually use, recognizing that I will be giving up a bit of critical sharpness. On the other hand, the very worst lens I own for smooth rendering is a Bausch and Lomb Ic Tessar, from the 40's (though it made a decent enough enlarging lens). That reinforces the point that while design archetypes may have tendencies, they can be and often were tailored to optimize for different qualities.
In medium format, nothing in my collection comes close to the Zeiss Jena Sonnar, 180/2.8 or 300/4, which is a vintage design even in the newer multicoated versions (it was made until about 1990). But in an MTF test, lenses of more modern design (particularly using computer methods) will score better at wider than about f/8.
For a lens to be critically sharp in the focus plane, it usually has to ruthlessly correct every bit of (at least) the primary aberrations. That means those out-of-focus disks will have a well-defined edge and an even brightness across their width. That's a lot better than the bright edges found on some lenses, if smooth rendering is your goal. But smoother than that may require giving up a bit on sharpness.
Here's the final point, however: Whiile bokeh is real, the evaluation of its effects is subjective. Thus, you are generally stuck with having to just try things until you find the lenses that make the images you like. It is not a two-dimensional effect, with only out-of-focus highlight rendering and aperture as the dimensions. There is also the shape of those disks (affected by aperture shape, and of no real issue if the disk fades at the edge but of primary interest when the edges are well-defined), and the relationship of that rendering to the size of the disk as controlled by depth of field or distance from the focus plane. And there are other, more subtle effects. Most photographers for whom this matters buy lenses until they find the ones that make the images they like and sell the rest.
Rick "suspecting that a purpose-built lens like a modern Cooke might be designed around these objectives directly, but bring your checkbook" Denney
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