Are you talking about the Dynamic Low Light mode? You can't turn that off. That is actually supposed to remove noise. The reason why the iPhone 5 images are so noisy is because the maximum ISO is now 3200 vs 800 on the 4S and higher ISO means more noise.
When there is enough light (eg outdoors) the ISO sensitivity is usually at 50 or 64. As it gets darker, the ISO sensitivity goes up and images become more noisy. Darker areas are also prone to noise compared to bright areas in the image.
When there is enough light (eg outdoors) the ISO sensitivity is usually at 50 or 64. As it gets darker, the ISO sensitivity goes up and images become more noisy. Darker areas are also prone to noise compared to bright areas in the image.