Using my D70, set on ISO 200, I shot some pictures yesterday in dappled shade (the sun coming down thru the leaves on the trees). Since the available light was somewhat limited, I got shutter speeds of 1/15, 1/30, etc. What is wrong with setting the ISO at 400? What do you sacrifice (if anything)? Does it really matter if you set the ISO at 200 for sunlight, and 400 for shade? Does that affect quality? I am confused about this. What real purpose does the ISO on a digital camera serve?