Noise, the digital equivalent of film grain, can be a challenge to overcome. It appears as an irregular, sand-like texture that, if small, is essentially invisible; if large, it can be unsightly and a distraction from your image. Noise can have color to it (chromatic noise) or only vary in brightness (luminance noise). (As with grain, this fine-patterned look is sometimes desirable for certain creative effects.)
Photographers generally don’t like grain in film, and the same is true for noise in digital photography. Camera manufacturers have been working hard to reduce noise at the time of capture, plus you can take some steps to reduce noise in your photographs as well.
Where Does Noise Come From?
The best way to battle noise is to understand what it is, why it happens and how to avoid or reduce it in the first place. Noise results from many causes:
Sensor Noise. Sensors always have some sort of noise, and reducing sensor noise has been one of the major research efforts of camera manufacturers. Such noise is due to several factors, including heat from the electronics and the way the sensor is put together. Sensor noise increases as the number of pixels increase within a given sensor size, although this is mitigated by the fact that noise-reduction technologies have improved as fast as megapixels have increased.
High ISO Speeds. Noise emerges when using high ISO speeds. Increasing the ISO in a digital camera is like turning up the volume control on a radio. When stations are weak, and you increase the volume, the static gets louder, too. Something similar happens when you increase your ISO speed.
Underexposure. On any camera, noise is more obvious with underexposure. Noise resides in the dark areas of a photograph. So when your entire photo is dark, and you make adjustments in Photoshop or another program, you’re allowing the noise to show up.
Heat. Sensors don’t like heat, yet they heat up with use. As sensors become hotter, noise tends to increase. This is especially true with long exposures. Increasing exposure time beyond a second or two makes your sensor work harder, increasing noise. That’s why many cameras have automatic noise-reduction features for long exposures.
Digital Artifacts. An artifact is anything that occurs in a photograph from technology and not from the scene itself. Noise is an artifact. Grain is an artifact of film. Digital imaging always has had trouble dealing with fine gradations, such as those found in the sky. That trouble with infinite gradations is because a pixel is either on or off. This sometimes shows up as noise in certain areas.
JPEG Artifacts. These artifacts are caused by image compression and reconstruction of an image when it’s opened in the computer. Higher levels of compression result in more JPEG artifacts.