This is a combinaton of screen shots with each ISO shot from ISO 50 to ISO 6400. The main objective is to verify that the noise is not necessarily proportional to the ISO setting. There is actually a significant drop in noise from ISO 160 to ISO 200. Also there is another drop in noise from ISO 320 to ISO 400, although not as significant as the case from ISo 160 to ISO 200. After ISO 400, the noise/ISO is more a linear function without the 'step' function behavior.

All shots were taken with lens cap on and with the camera on M mode. You can see the ISO of each screen shot at the top of each file.