I have a simple point-and-shoot camera that takes satisfactory pictures when the picture is filling the computer screen. The pictures are shot with auto ISO and full daylight. But when I look at the picture at 100% - it's grainy. At first I thought of just cleaning the lens before posting this question, but several answers on this site suggest not cleaning a lens unless there's a good reason, so as not to scratch it.
So my question is: is this possible at all, that a lens that isn't completely clean (or is scratched?) will degrade the image quality without a large-scale degradation of the picture, such as a fingerprint seen on the image, or dirty smudges etc.? (Assuming it's not the ISO etc. at fault)