Originally posted by Nass Lowell, fascinating. Quick aside, you say "of this era". Would you happen to be able to give an approximate date when Pentax lenses started being aprochromatic? Many thanks & apologies to OP.
It began in the late 70's and early 80's with the proliferation of color films especially print films, and displacement of B&W
There was from memory a lot of discussion about it then.
The degree of CA changes lens to lens, and is really obvious in some longer lenses like the SMC 300 F4. I have not really gone looking for it in the SMC 135F2.5 (K mount) but I am sure I can find some.
The whole issue is that part of the bokeh of B&W included the CA, as shading, but when viewed in color becomes objectionable in a hurry.