I went to the zoo today to take some pictures and I ran into a problem I have experienced a few times before. Some of the pictures were underexposed (it was so bright out I had a hard time reading the meter while looking through the viewfinder) and when I opened them in DPP and adjusted the brightness to lighten them up, there was quite a bit of noise. My ISO was set at 100. Is this what happens when dark photos are lightened so much, even when they're taken at 100 ISO? I thought that high ISO would be noisy but not low.