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Definition: depth of field

The area in an image from front to back that is in focus. The smaller the aperture (the larger the f-stop number), the more objects are in focus both near and distant. The wider the aperture (the smaller the f-stop number), elements in front of and behind the object in focus appear soft or blurry.

Set a Mood
In both moving and still pictures, depth of field (DOF) is widely used to call attention or create feelings. By focusing on one element in the image and leaving the rest blurry, the audience is drawn to that part of the frame. In addition, making surroundings softer or foggy can eliminate unwanted background objects that distract from the subject of the picture. See f-stop and focal length.

Quite a Difference
Changing the f-stop from f/29 (top) to f/4.5 (bottom) turns all the unwanted objects in the background into a blur.