Overall photo quality produced by the S640 is very good. Many cameras in its class suffer a significant dip in quality when they use any sensitivity above ISO 200. The S640 is actually good to ISO 400, though there is some visible noise and increased softness. The camera lets you limit the auto ISO range to either 100-400 or 100-800. If you're in daylight or bright conditions, I recommend locking it down to 100-400. It performs OK up to ISO 800, but there's definitely increased softness and noise/artifacts. The S640 can shoot at full resolution up to and including a sensitivity of ISO 6,400. However, everything from ISO 1,600 up doesn't look good because of color shifting and yellow blotching and the near absence of detail. So while you can, in fact, keep shooting in low-light conditions, you probably won't be thrilled by the results.