Smooth means (relatively) free from noise (as in smooth tonal gradations). D810 images (correctly focused) are clearly more detailed than D5 images (both at low ISO); at the same time they have less noise at low ISO settings. If you prefer measurements you can just look at dxomark they'll show that mid-gray SNR at ISO 64 for D810 is 46.3dB and for ISO 100 on the D5 it is 44.8dB; dynamic range 12.26ev vs. 14.76ev. It's easy to find evidence for the extra detail in D8x0 family images for example look at dpreview's studio comparisons. My subjective observations in this area of sensor performance of these particular cameras align with dxomark's measurements. Usually when measurements are correctly done they agree with subjective observations from a person who is experienced in evaluating image quality and understands what they are evaluating. What gives the D5 the edge is its autofocus system, however, if one is shooting subjects which are not moving a tripod and live view is what I use, and in that case the D5 has no advantage compared to D8x0 family cameras, unless of course the D5 is the camera in your hand when the subject presents itself in good light and alternatives are not available.