I was out takeing pictures of a friends little girl the other day and it was around 7pm. We photographed some of the pictures under a few trees where there wasn't very much light. After I downloaded some of the pictures, I noticed that the photos tended to look very grainy and noisy. I use a Nikon D200 with Iso at 160, Whitbalance on Auto and and Exposer- 1/160 f/4.5. Can anyone tell my why the pictures turned out to be so grainy and noisy? Should I change my whitebalance and Iso settings? Thank you for your help.