Circular Banding is more likely vignetting correction.

just did a test just to eliminate any theories that the banding is caused by either lens, sensor, or both.

the answer is neither. what it could be is vignetting correction applied on the raw file during image processing in the NX200 camera. I tried putting a tape over the contacts of the NX lens as was suggested by a forum member and took some shots. the images turned out negative with the banding which suggests that it is indeed a form of further processing applied. now this type of processing is absent on the NX100 and is only applied on the NX200 with NX lenses. this has nothing to do with the optical quality of the lens but rather lens profile of the lens communicated thru the contacts before processing is made. the absence of lens profile (by taping or cover) would prevent such application from happening.