This is a test of the camera's noise levels. The higher the signal to noise ratio, the greater the difference in strength between the real image data and random background noise, so the 'cleaner' the image will look. The higher the signal to noise ratio, the better.
Nikon D5 signal to noise ratio charts
JPEG signal to noise ratio analysis: Up to around ISO1600 the D5 lags a little behind the D4S, but above that figure it starts to take the lead, indicating that the higher sensitivity images are cleaner. It's not until ISO25,600 that the D5 overtakes the 1DX Mk II, but the Canon camera doesn't fall far behind, and produces the best results at ISO409,600.
Raw (converted to TIFF) signal to noise ratio analysis: Interestingly, the raw files (after conversion to Tiff) don't compare quite so well to those from the D4S, perhaps indicating the downside of increasing the pixel count, but also suggesting the improvements that have been made with in-camera JPEG processing. The Canon 1DX Mk II beats the competing Nikon cameras at all but the lowest sensitivity values. DXO Analyzer was unable to analyse the raw files taken at the D5's upper sensitivity values.
Sample Nikon D5 ISO test results
The signal to noise ratio charts use laboratory test equipment, but we also shoot a real-world scene to get a visual indication of the camera's noise levels across the ISO range. The right side of the scene is darkened deliberately because this makes noise more obvious.
ISO 200: Click here for a full-size version
ISO 6400: Click here for a full-size version