[Scythels]

You can't have your cake and eat it, everything comes with a price.

Yes modern lenses are ususally better performers but this comes with a price. the aspherical elements tend to deteriorate the bokeh, but would you prefer a lens without aspherical elements ? all depends of your preferences.

more elements might have an effect on color, but newer lenses have better coating so the overall result is generally much better.

Would you prefer using an old yashica lens from the 50s over Canon 50mm  f1.2 L ??

Categorically speaking the "onion ring bokeh" isn't because of aspheres, but because of CNC manufacturing methods. 


Oldschool traditional methods are what is known as "full aperture" grinding and polishing.  They hit the entire surface at once, evenly, so they do not leave any small / local artifacts.  You can do up to about 6 elements on a single machine and the machinery is cheap and compact, so it's great for volume manufacture. 

 

The downside is that you need master opticians with often 25-35+ years experience to monitor things, since it is a "blind" process, and they are a dying breed.  The surface quality produces can also not be too hot, and you can't make anything but spherical shapes. 

With CNC "anyone" can operate the machine and the quality of surfaces produces is ~4x better.  The cost is also ~2x higher. 

 

Unfortunately periodic error in the servos prints through into the parts, since CNC is "sub aperture" polishing and grinding, meaning it works on only a small part of the surface at once.  The result is a "new breed" of manufacturing errors known as mid spatial frequency errors.  The rings are a form of mid spatial frequency error.  They came first with aspheres when CNC was not popular for spherical parts, but now because they can hire almost any joe schmoe to run the machine instead of finding master opticians, it has become vogue for all parts (which is imo very good). 

 

You can see CNC grinding/generation simulated here -  https://www.youtube.com/watch?v=4_etSKB0ueY

 

And you can see a (lengthy) webinar on optical surface spec with a great deal of MSF error discussion here - https://www.youtube.com/watch?v=mcl3PppDu5o

 

Dr. Degroote Nelson teaches the course OPT 243 - Optical Fabrication and Testing at the University of Rochester and is director of R&D at Optimax, a very large optics manufacture in the US.