This is a very boring but important test if you want to find out how the noise levels in the darkest blacks will increase when you crank up your ISO.
The FS700 provides very clean blacks up to 1600 and is absolutely usable up to 3200. 6400 is the cut off if you don't want to compromise too much but it still looks nice since the natural structure of the noise. But if your coverage depends on it you can crank it up all the way to 16000. We should call this Night for Day!