The signal to noise ratio is the ratio of random digital noise to real image data. The higher the signal to noise ratio the better. This inevitably drops at higher sensitivity settings, but the drop-off in performance does vary from one camera to another.
JPEG signal to noise ratio analysis: The X-Pro2 delivers a very good set of results that closely match those from the Panasonic GX8. Interestingly, the Sony A7 II can't quite keep up and delivers JPEG images with a similar noise level to those from the Olympus OM-D E-M5 II.
Raw (converted to TIFF) signal to noise ratio analysis: The raw files tell a different story, however, and here the X-Pro2's images prove significantly noisier than the rest, while the Sony and Panasonic are the best.