Adobe Photoshop CS5 – 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 stock-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).
Tip provided by Bryan O’Neil Hughes – Layers Magazine