Say you’re tuning a radio to a station with a weak signal. You turn up the volume and hear static. Why? By amplifying the signal, you’re also amplifying unwanted stray electronic bits: audio noise.Electronic noise in digital photos has the same cause: Amplify a weak signal from an imaging sensor too much, and you also amplify stray electrons in the sensor or elsewhere in the circuitry. It shows up in your pictures as coarse grain (sometimes okay) or color mottling (often pretty ugly).