The image captured by a digital camera is created by the sensor in the camera, which consists of millions of individual cells, each representing a "pixel", that converts the "analog" light passed through the lens into digital information about the light. Noise is created by the electrical interference neighboring sensor pixels have on each other.
In trying to think of an analogy from real life, what came to mind for me was a call-center where many service representatives are handling individual customer calls. Each is trying to focus just on their call, but the conversations of those around them intrude somewhat, and the more people packed into the room, or the more calls being taken at the same time, the greater the level of interference one service rep has on another. And then as the noise gets louder, people have to talk even louder, compounding the problem.
Just like the level of interference from others working the phones in the call center is affected by such things as the number of calls happening at once, the distance apart each service representative is from the others around them, the "energy levels" of the individual representatives, etc., so too the interference that causes picture noise in a digital image becomes more magnified by variables such as the number of pixels on the sensor, how tightly packed they are, the size of the sensor itself, the length of the exposure, and the ISO setting at which the photo is taken.
If you click on my photo to the left with a lot of blue sky, you will hopefully be able to see the noise in the blue sky. Rather than being smooth, continuous blue sky, you can see that it is somewhat "grainy". This is an example of digital photo noise.
What steps can you take to reduce noise?
The steps you can take to reduce noise can be broken into two categories: first, how you take your picture ("in-camera"), and second, steps you can take in processing the image with software after the fact ("post-processing"):
- Shoot at the lowest ISO setting possible for the light conditions since the higher your ISO setting, the more noise is generated as you "amp up" the electrical activity of the sensor
- Shoot at the fastest exposure possible for the light as the shorter the exposure, the less time for noise to be generated
- If you are shooting in low-light, use a flash if appropriate
- If you are finding that your exposure is getting longer and you aren't shooting for depth of field, then take the photo at a lower f-stop since this will let more light through the lens and allow for a faster exposure
- In low light shoot with a tripod so that you can take the picture at a lower ISO setting without worrying about blurring
- In your "post-processing workflow", always look for obvious noise in your images and include steps to reduce it as much as possible. Photoshop Bridge and Adobe Lightroom, for example, have noise reduction sliders to help reduce noise. There are also many noise reduction plugins for Photoshop available on the market. You can typically get a free trial of most of these and and take them for a test drive.
No, sometimes noise is introduced deliberately in image post-processing to either recreate the look and feel of old photographic papers and high-speed films, or to create digital artwork known as "grunge", where noise is also an integral part of the overall look. For example, I deliberately added noise to this photo of an aloe stalk.