Grainy IS how cameras adjust for lighting conditions. If they didn't adjust for lighting conditions, you'd either have almost black pictures in even overcast conditions, or washed-out white in direct sun.

People don't realize how vast a difference there is in lighting between full sun and even shade on a sunny day or overcast. It's a huge difference. We have incredible cameras in our eyes, and we seldom think about how good they are.

Digital cameras generally open their lenses up farther to get more light - this isn't possible on cheap fixed-lens cameras like the OS, and in any case only goes so far - or they slow their shutter speed down - they can do this, but not below 1/30th second if they're shooting 30 FPS. The only other trick they have is to just boost the image in software - essentially they're just multiplying the level of each pixel by some amount to brighten the image. The problem is that also multiplies the low-level noise inherent in the camera's sensor which is normally too low to see. That's what the grain is, it's sensor noise, and the REASON you see it is that the camera *IS* adjusting for low level. The root cause is that the camera has a very tiny, cheap sensor. You eliminate noise by using better and larger sensors, but that costs money, especially since bigger sensors require bigger and better lenses, and the lens that's in most cameras costs about a nickel.