Shutter-priority automation does the opposite. It lets the photographer set the shutter speed while the camera selects the aperture for proper exposure. Many cameras offer programmed exposure, in which the camera chooses both the shutter speed and the f-stop. All automatic exposure systems rely on readings from a built-in light meter. The camera's CPU interprets these readings according to a metering pattern, a set of instructions that tells the CPU how to react to different amounts of brightness in specific parts of the scene.
Some cameras offer a choice of metering patterns.
Using the readings and a metering pattern, exposure systems can compensate for much unevenness in lighting, such as a scene with a bright background behind a dark subject. Automatic focusing, also called autofocus, adjusts the lens focus automatically to provide sharp photographs. Sensors measure the angle at which the infrared beam returns to the camera or how long it takes the ultrasound wave to return. The system uses the measurement to determine how far away the subject is and where the lens should focus. A passive autofocus system analyzes the sharpness of the image entering the lens.