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Noise refers to pixel-level variances in color value that reduce the clarity of an image. Static is an extreme example of noise, resulting from the complete absence of a signal.
Noise in digital images is somewhat analogous to grain that appears in film photography. The major difference is that noise tends to take on primary RGB hues, rather than the more aesthetic hues of the dyes in film grain crystals. A noisy pixel tends to be brighter than its surrounding, while a noisy grain tends to be of the same brightness. Noise differs from dust in that it is an intrinsic property of the imaging process, rather than an external object.
The presence of noise or grain in photography occurs in a similar way: increased sensitivity (ISO) causes a sensor pixel or grain crystal to pick up more light than its neighbors. In extreme situations, photons may only strike certain pixels or grains, being indetectable elsewhere.
The actual mechanics vary:
- A pixel may gain noise due to electrostatic charge present on the sensor being interpreted as a visible phenomenon. Also, the process that amplifies the sensor count and translates it into a datum can aggravate what would be a minor difference at lower sensitivity.
- A larger film grain can collect more light than a smaller grain. Therefore, higher sensitivity film has coarser grain than a lower-speed film. In addition, variances in grain size or poor production quality can cause some larger grains to be interspersed with finer ones.
Noise in Photoshop
In many cases, noise is unwanted, and many features in Photoshop are geared towards the reduction of noise.
In other cases, noise may be desired. The Add Noise filter is specifically geared towards increasing the occurrence and strength of noise.
- When transplanting an object into another image, it helps to match the noise levels between both elements. Noise in the object can be altered more easily than noise in the scene.