Using NEF files, high ISO shots look great up to 1600, good - after luminance noise reduction - above 1600 and up to 3200 for screen and in prints. Beyond that, everything falls apart. The camera is working well therefore and exactly as designed. That's all with a shutter speed of 1/60 or faster. Drop the shutter speed below that and excessive noise starts showing up below 3200. Drop the shutter speed below 1/2s and things get terribly noisy at 1600. So the D5200 is no different from all digital cameras in that really high ISO works best at the fastest possible shutter speed at max aperture of the lens for a given focal length.