Sharpness & DOF
|F/2; DC/0: DOF/2cm|
|F/2.8; DC/0: DOF/4cm|
|F/4; DC/0: DOF8cm|
|F/5.6; DC/0: DOF/10cm|
|F/8; DC/0: DOF/12cm|
The best sharpness is really started from F/4.
In the sense of DC-Nikkor lens, BOKEH of a image does not equal to out-of-focus.
At F/2, DC/2; F/2.8, DC/2.8; F/4, DC/4, the lens adds a very slight BOKEH to the overall image. It is a nice setting for portrait.
At F/2, as DC setting increase the overall BOKEH also increase. It is really used for special effects.
At F/2.8, DC/2 moves the BOKEH forward; DC/4 moves the BOKEH backward, DC/5.6 further backward.
At F/4, DC/5.6 moves the BOKEH backward.
Thus, the best aperture to use is F/4 at DC/0 for sharpness; F4 at DC/4 adds some soft effect to the image; F/4 at DC/5.6 increases the background BOKEH.