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