At 12mm and maximum aperture, sharpness is already excellent across the frame, and even outstanding in the center. At this focal length, diffraction appears to be the limiting factor on clarity, with the performance of the lens reducing as the aperture is stopped down.

18mm show similar performance, with outstanding sharpness in the center of the frame at maximum aperture, although excellent sharpness isn’t achieved towards the edges of the frame until the aperture is stopped down to f/5.6 for this focal length.

At 24mm, sharpness remains excellent in the center of the frame at maximum aperture, and is still very good towards the edges of the frame.

12-24mm optic performs well, delivering excellent, or even outstanding sharpness from a compact, lightweight lens that doesn't cost the earth.

http://www.ephotozine.com/article/samsung-nx-12-24mm-f-4-5-6-ed-lens-review-26895