alt Hacker News> The 105mm 2.0 DC and slightly older 135mm 2.0 DC [...]
I almost never change the defocus control on the 135/2.0 from neutral, yet I believe the lens makes it almost impossible to take a bad portrait. The only reason to keep a DSLR around instead of a mirrorless is that this lens has an old-school camera driven auto-focus, which Nikon's F-to-Z adapters don't support. While manually focusing that lens can be done, the shallow depth of field makes it very challenging for moving subjects.
It's definitely worth messing with. The reason it has a F and R side (with aperture markings) is that a lens will be optimised to either provide ideal bokeh at close or long focus (long focus bokeh being things blurred in the foreground closer to the camera). Try setting it for example to F 2.8 real aperture and then I believe move the DC ring to R 2.8 or 4 and you should get a very pleasant creamy blur. Or for a slightly sharper portrait move to neutral or R 2.0