Noise is an imaging artefact, an error of capture that has nothing to do with the actual scene being photographed. It resembles graininess in film, but its causes are different. It increases with higher sensitivity settings, just as graininess is stronger in high speed films. Caused by many factors, including heat, the nature of the electrical signal and activity in the sensor itself, the outcome of noise is always the same, irregular. Noise usually appears as speckles, which can be black, coloured, or bright, depending on the scene and the exposure. Noise is generally more evident in lower mid tones and shadows, and in areas of even tone.
There are two situations where you are likely to find noticeable noise. The first is when you chose a high sensitivity setting, and the second is when you make a long exposure. When we increase the sensitivity, by using a higher ISO, we are amplifying the signal to the sensor, this process tends to increase noise further by making it more visible.
Several things contribute to noise, but the major cause of it is when then there are not enough photons of light striking the receptors in the sensor, this then causes a sampling error.
Because noise tends to mask detail, it is generally regarded as a flaw that should be avoided or reduced, however, it does have value, provided we introduce it only when and to the extent that it’s needed. Noise can help give texture to high resolution images. The important point to remember about digital noise, is that you have to decide which structures in the image are real detail, and which are noise.
There are different kinds of texture in the garment, folds, vertical ribbing and mottling. Think about the mottling on clothing. Is this noise, or is this part of the pattern? it could be either. Distinguishing detail from noise is ultimately subjective.
Digital Photography Master Class, Tom Ang, Dorling Kindersley Limited, London 2008. Page 18