You get called dark current (thermal noise) on CMOS and CCD sensors, and the signal-to-noise ratio goes down as you decrease the exposure time. You see the effe

You get called dark current (thermal noise) on CMOS and CCD sensors, and the signal-to-noise ratio goes down as you decrease the exposure time. You see the effect when taking a low light photo, the graininess is from the SNR being very low while the noise is showing through. You can smooth this out quite a bit by combining burst photos (see Cortex Camera app) as the noise is random, but you always get better results with more light for each exposure.