Understanding Noise in a Digital Photo
Digital camera sensors have varying degrees of sensitivity to light. The more sensitive the sensor, the better it can capture images in low light levels — but a sensitive sensor also produces noise.
Your camera increases its sensitivity by amplifying the signal captured by the sensor, so photons that were almost too dim to register are electronically beefed up to become full-fledged pixels.
Unfortunately, at the same time, non-photons are mistaken for true pixels and assigned pixel-hood. These bogus pixels are what photographers call noise. Noise appears in your pictures as multi-colored flecks that look like grainy, irregular dots.
Many cameras let you specify the sensitivity, and you can increase the sensitivity to give you a faster camera. But upping the camera sensors’ sensitivity usually increases the amount of noise in the image.