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Random coloured pixels that appear in dark or shadow areas when the light levels are below the camera's CCD sensitivity range. Noise is also often seen from scans made using a scanner that doesn't have a wide enough dynamic range to cope with the shadow and highlight areas in one scan. Some image editing programs have a Noise filter that adds a grain pattern to the image to make it look more like a natural photograph taken using film.
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In digital photography this is the amount of correct picture information to unwanted electrical interference. The S/N ratio should always be high so that good quality noise free pictures are produced.
Used to indicate the light sensitivity of a film. Digital cameras also use the ISO rating to indicate the CCD sensitivity. The standard rating is ISO100 and as this is increased it means that faster shutter speeds can be used. When the ISO is doubled, it doubles the available shutter speed. The drawback for digital cameras is that increasing the ISO increases the amount of digital noise in an image.