On Sun, 7 Feb 2016 13:40:45 -0800 (PST), RichA
wrote:

Remember when people were pushing cameras to the limit, shooting images 5-6 stops underexposed at the highest ISO to see if they could form some kind of image? That happened about 7 years ago and I think one model (Nikon D3?) managed to show something at the equivalent of 1 million ISO. They should be capable of going beyond that now. It's not to produce anything worth keeping, just an exercise to see what the sensors can do. You'd get something like Fox Talbot's first shot of cityscape or window. A high contrast image helps.

My D810 can produce images that are usable on the web at ISO 51,200.
Not great for printing, but when reduced they're OK.

The GX8, about half that, same result.