Grain's Anatomy

Film grain has complex characteristics, both in spatial structure and in value/intensity. Many understand the basic principle behind how film works (in simple terms, light sensitive crystals - or grains - causing variations in density depending on the amount of exposure to light), and therefore why grain exists and why it tends to have the shape it does (lots of overlapping crystals/grains). What isn't immediately obvious to many is how the intensity of the grain is influenced by the nature of film.

The behaviour of film, and the variations in density - caused by the grains - can be measured and from that, complex graphs can be produced to somewhat quantify its nature, such as the following:

Each film stock type has its own specific characteristics, but given all the complexity, there's one main overriding factor: the basic log-exposure nature of film. Its influence is enough to almost overshadow every other aspect of the grain intensity.

Shown here is log data from the bottom section (the grey patches) of a ColorChecker chart:

You can see that the grain intensity (or variation in density) is similar in the dark and brighter areas, tending to a little higher in the dark areas (as per the graph above). What happens after a log to lin conversion?

It becomes immediately obvious that the grain intensity is now much greater in the brighter areas than in the dark areas. In effect, what happens is that in linear space, the intensity of the grain is directly proportional to the value it's being applied to. That is, a value twice as bright will have twice the intensity of grain. That's a bit of a generalisation, but it's an appropriate approximation to what goes on. Grain on a simple black to white linear gradient generally looks like:

Zooming in closer on the grain, you can see how the log nature expresses itself not just in the overall intensity, but in the basic shape of grain itself, here shown as log data:

After a log to lin conversion, that becomes:

You can see that even within the grain itself, after conversion to linear, the grain extends a lesser amount below the main/central value, and more above.

This behaviour has some implications. One issue that would occur is when a series of frames are averaged together to produce a clean plate with grain reduced/removed. If linear images (ones that have already had a log to lin conversion) are averaged together, since the grain extends more above and less below the main image value, the average will be higher than the real main/central image value. It is better to average the log images, where the variation in grain is linear, and then apply a log to lin conversion to the averaged result. The differences aren't huge, but they are definitely there, and could certainly affect black levels, where it's more likely to be noticed.