The electronic noise generated by the sensors in digital cameras is, in very broad terms, the equivalent of grain in traditional film photography. Each cell on a digital camera sensor is very small, particularly when the sensor is physically small and and also has a high megapixel count. Compact camera sensors are typically of the 1/2.3 inch type but may feature over 8 or 10 million individual cells. These are contained within an area of about 28 square millimetres. Individual light-sensing cells are consequently so small that in low light conditions they collect only a few thousand photons during a typical exposure. The electrical signal produced by a cell under these circumstances can then be subject to random statistical fluctuations in photon density, resulting in a low signal-to-noise ratio. In physically larger sensors the individual cells are also much larger and therefore collect a much higher number of photons. This reduces statistical variation and produces a higher signal-to-noise ratio. In good light, such as normal daylight, huge numbers of photons are collected by even small photocells and the problem is much less significant. The noise problem worsens when high ISO settings are used. A high ISO sensitivity implies shorter exposure periods and hence fewer photons collected by each cell, lowering the signal-to-noise ratio. Images taken at high ISO settings are consequently always more noisy than those captured at lower settings.
A second type of sensor noise may become a problem during long exposures - typically those over a few seconds. Irregularities in manufacturing processes and errors generated during signal transmission may result in individual cells having slightly different responses to light. Single pixels may then be too bright or dark. The charge built-up during a long exposure makes manufacturing imperfections of this nature more apparent. However the pattern of visible errors will be consistent from frame to frame and can be corrected by applying a custom-designed masking filter during image processing. Modern cameras incorporate this masking process.