7Noise in Digital Photography

Noise is one of the intrinsic factors which limits the ability of a photograph to reproduce a scene as accurately as possible.

In film photography, the main noise affecting images is encountered during the formation of the latent image (physical absorption of a photon by the sensitive material used). It then becomes apparent during the chemical transformation of this material during development: rather than being limited to the photon impact point, the image spreads to a small, compact local structure. This is known as speckle in a film photograph ([KOW 72] p. 95). Speckle consists of the production of uniform areas of gray by the juxtaposition of very small “atoms” of varying density, which constitute the active matter of the emulsion (generally silver halide). When the noise is stronger, these atoms are larger, and the variance of their gray level increases; they thus become visible, even in moderate enlargements. The size of the grains in mass-market products typically varies, from 5 μm to 50 μm in diameter. This constitutes an increasing function of film sensitivity (measured by the ISO number, see section 4.5), but also of the potential contrast of the film.

In digital photography, the reasons for noise are very different, and result in very different visible impairments. Noise does not directly affect image resolution in this case, but rather the gray level value of each pixel. Thus, noise in digital photography has a very similar meaning ...

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