Grain size represents the size of the film frame we’re emulating - 35mm film will have a very fine grain structure, while 8mm film, being smaller, will have larger, more prominent grain.
The curve you see is the natural response for the grain in your selected film stock - shadows start at the left, through to highlights at the right, just like a histogram. The default curve is native to the film stock, but you can adjust the appearance of the grain in the shadows, midtones and highlights just by clicking and dragging points in the curve.
The other film grain settings are available in the plugin settings.
Size is the size of the grain. Moving the slider to the right will increase the area of each grain particle.
Softness is softness, as you’d expect. Film is naturally softer than digital sensors, so when creating the film look this control allows you to soften your edges.
Strength is the overall amount of grain applied to your image. Move it to the right and you will get a strong grain effect, and taking it to the left takes away your grain all together.
Finally, there is grain saturation. All film grain has certain colour characteristics and you can use this slider to boost that colour or take it away completely and have monochrome grain on a colour image.
