ISO auto logic and D4S question on ISO
The camera will go to your minimum shutter speed first with ISO at 100.
If still underexposed it will raise ISO up to your max.
If still underexposed it will lower shutter speed below the minimum.
I recommend you shoot at various ISO to determine how much noise you find acceptable. I don't worry about it too much and keep my max ISO at 25600 because I prefer a grainy image over a blurred one. If I'm reaching these limits I'll switch to M anyway.