Add grain to bring consistency to composites
One of many changes to Camera Raw 6 is the ability to add grain. Grain can be used to add warmth to an image, hide blemishes and mimic film stockâ”but can also be used to establish consistency between images of varied ISO (or "film speed"). Consider the case of a shot taken midday and another at sunset; compositing the midday image onto the sunset will look artificial due to mixed texture, but a sprinkle of additive grain will bring consistency and believability to your composite. Best off all, the results are editable and non-destructive (when used as smart objects).
Adobe Photoshop CS5
By Bryan O'Neil Hughes