What is the difference between digital high ISO noise and film grain? Why does one "eat detail" and the other does not?
The size of the grains in the film varies depending on the film sensetivity. The more sensetive the film, the larger the grains. Digital noise is always the size of a pixel, regardless of the ISO setting.
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.
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.
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.
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.
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There's also the issue of chroma (color) vs. luma (brightness) digital noise. People generally find chroma noise more objectionable because it appears less natural; this is why noisy photographs sometimes work better in a B&W conversion. The better noise reduction algorithms can also address one or the other selectively.
I believe that film has pretty much only luma noise, which is one reason people sometimes find it less objectionable than digital noise.
The biggest difference is the patterns in the noise. Film grain is caused by the grains of silver present in the film, and are not in a consistent pattern.
ISO noise is caused in the digital sensor and is pixel based, and therefore in a pattern.
Some feel that film gran is more pleasing because of the inconsistent pattern in which the noise occurs.
Higher ISO film tended to have more grain; and higher ISO digital shots exhibit more noise - a similar cause, but the visual appearance is different.
Digital ISO noise is related to the size of each pixel, as the noise is per-pixel (so the more pixels you have, the less obvious noise is when viewed the same size), whereas with film, the noise is per crystal - you need larger crystals for higher sensitivity.
One analogy I've used in the past to demonstrate this is to ask several people to time with a stopwatch how long it takes a car to drive around a car park, and then to time how long a person takes to do the same journey - because the person is slower, the margin of error is a smaller in proportion to the overall figure, even though different people will give timings to within a few seconds of each other.
I did a 5 part series on Noise Reduction and reviewed the leading products in noise reduction (which many of those software companies have called one of the best and most comprehensive reviews of noise reduction software).
I'd suggest you check it out and see.
Here's the thing though, not all noise is bad. An image completely free of noise is generally going to look plastic like. Noise reduction can reduce texture. Finally, there are actually products designed to add noise back in (like RealGrain) because it can be preferable to have some noise when printing. The thing is you generally want to remove the bad noise (chroma noise) and then dial back in the good noise if you want it.
This is humongous oversimplication so go check out the article for more details. There's also discounts on the noise reduction software too.
Finally remember, noise is subjective - movies like Saving Private Ryan digitally added a lot of noise to get a classic "film grain" look, so it isn't necessarily an evil thing.