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PS filters: New layer --> fill with 50% gray --> add noise (about 10%, gaussian, monochromatic) --> Gaussian blur (radius about 0,5px) add noise (about 5%, gaussian, monochromatic) --> Gaussian blur (radius about 0,2px) --> set layer mode to Overlay and set transparency to 20-40%. Everyting depend on your picture pixel size, If I've got too big ( then grain is almost not to notice ) I resize grain layer to about 150-200%. And transparency: sometimes is enough 20% but sometimes it's good at 60%, just try. It's nice when you flat (into new layer: Ctrl+Alt+Shift+E) everything and then use unsharp mask filter over this, the grain is sharper, more intense and get better visual integration with picture below.