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Modern cameras are generally so advanced that when set to auto nearly every picture is exposed correctly. That said, a camera never knows exactly which image you intend to take. Should the landscape in the foreground be in focus, as well as that in the background? Should your photo of a penalty kick have the soccer ball in focus or all of its surroundings?
Digital system cameras, and some advanced compact cameras, allow you to manually set the aperture and the shutter speed.
Selecting only the aperture: You may choose between a small, medium, or large aperture and let the camera determine the appropriate shutter speed. This will allow you to have full control over the depth of field in your photo. A small aperture increases depth of field (where the foreground and background of an image are be in focus), a large aperture provides a shallow depth of field (where your subject is in focus but the foreground and background not). In this way you can actively determine how you’d like your photo to turn out.