For bokeh (out-of-focus blur rendering) tests, I usually like to have a second lens for comparison. However, I currently lack another 40mm equivalent lens to compare with the DP2 lens, so I'll just provide some bokeh samples to give an idea of how the DP2 lens renders out-of-focus blurred areas. Unlike lens sharpness, one can get a sense of bokeh from images resized for the web. I'll point out a few other concerns in specific examples.

F/2.8:



While the DP2 lens has decent flare resistance, it flares more than the DP1 lens in my experience. The above photo shows flare with a stong off-axis light source. The hood was employed, and no filter was present. You can also see a blue-green discoloration on the wall in the upper right aspect of the frame.





F/4:



F/2.8:



F/4:



Looking at the right side of the frame, you can see how points of light are rendered sharply as heptagons at this aperture. Some blue-green discoloration is also evident in periphery on the wall.

F/5.6:



There is the occasional doubling of elements, but overall I find the DP2 bokeh to be neutral and generally pleasing.

The issue of blue-green drift in the corners/periphery was present with the DP1 and effectively addressed in a Sigma Photo Pro update. As of yet, the issue persists with DP2 files but is rarely a major issue and can be addressed in postprocessing. A few examples of color in DP2 photos from a recent day out with the family are shown below (click to enlarge).