Film grain and halation
Grain is one of four film-stock structures, applied at a size relative to the frame rather than to the pixel grid. Halation is a red or orange bloom that appears only past a brightness threshold you set. Both render in the viewfinder and in the saved file through identical maths, so what you frame is what is written.
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What the grain control actually does
Four film-stock structures on a Fine, Medium and Coarse scale, with crystal size and clumping you can dial in separately. Clumping is what separates film grain from digital noise: real grain clusters, and evenly distributed speckle reads as a sensor problem rather than as a stock.
A preset can bring its own grain settings with it — the amount, size and frequency fields Lightroom writes are read and applied — and the app's own controls sit on top of that.
Why grain is sized to the frame
Grain applied at a fixed pixel size looks completely different on a 12 MP frame and a 48 MP one: the same setting is coarse on the small frame and invisible on the large one.
So the structure is scaled relative to the frame instead. A look you set up in the viewfinder holds together when the capture runs at full sensor resolution, which is the only way a live preview of grain is worth anything.
What halation actually does
Halation is the red-orange bloom film gets where a bright highlight scatters back off the base behind the emulsion — street lights at night, a window in a dark room, a rim light on hair.
It is not a global glow. The effect applies only past a brightness threshold, which is what keeps it on the highlights that would really bloom instead of hazing the whole picture.
Setting the threshold
Lower the threshold and more of the frame blooms; raise it and only the brightest specular highlights do. Red is the more filmic of the two colours; orange reads warmer and holds up better against tungsten light.
Because it renders live, this is a control you set by looking rather than by guessing — which is the whole argument for putting the grade in the viewfinder in the first place.
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