Concentric rings also showed up in a test designed to test compressed NEF quality in highlights. Basically an unfocused white piece of paper (unedited original looks unitform light yellow-white) where levels were edited manually in an "auto-level" fashion to spread the very few tones over the histogram. The subtle rings that unexpectedly showed up must originate in NX since the steps of the original tones in the image are much larger after editing levels. (The overall tones look smooth, however that is because NX interpolates across the original steps; the histogram of the edited Nef file shows interrupted lines.)