What is the difference between digital high ISO noise and film grain?

What is the difference between digital high ISO noise and film grain? Why does one "eat detail" and the other does not?

Answer

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  The size of the grains in the film varies depending on the film sensitivity. The more sensitive the film, the larger the grains. Digital noise is always the size of a pixel, regardless of the ISO setting.

  
  


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  Film grain is color neutral, as it consist mostly of luminance differences. Digital noise consists of both luminance and color differences, and is most visible in the blue color channel.

  
  


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  In the more recent digital cameras the digital noise is quite even. In earlier models the noise had more banding and patterns. The film grain doesn't have any banding or patterns, so it's seen as pure noise. If the digital noise has any banding or pattern, the brain can easily pick that up, and that is more disturbing than pure noise.

  
  


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  Neither grain nor noise eats detail. It's noise reduction that eats detail, as it can't tell the difference between small details and noise. Noise reduction is used on digital noise, but it can also be used to reduce film grain.

  
  
  

There is an example below. On the left is the film grain from a Kodak Gold ISO 200 film. On the right is the digital noise of a Canon EOS 5D Mark II @ ISO 3200. Notice the blue noise in the dark areas in the right image.