A camera sensor is made up of millions of tiny light-sensitive sites, each of which measures brightness and colour. When the scene is very dark, these sites don't receive much light to work with. The sensor still has to produce a complete image, so the camera boosts the signal electronically. This amplification makes the picture brighter, but it also exaggerates the sensor's natural imperfections. The result is digital noise, which appears as grain, static or patchy colours.

The most common form is luminance noise. This looks like grain or sand across the image and tends to show up most strongly in the darkest areas. It can make walls look patchy or dirty, or give flat surfaces strange mottled textures.

You'll also see chromatic noise, which usually appears as tiny dots or smears of random colour. These can sometimes cluster into pink, blue or green patches that flicker between frames, especially on video.

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