In most systems, sensors behind the lens measure the contrast of lines or edges in the subject. This contrast is highest when the camera is sharply focused on the subject. The system adjusts the focus to achieve the maximum contrast. Some autofocus systems can sense if a subject is moving. These systems make adjustments so that the subject is in focus during the instant the shutter is open.
Types of cameras Fixed-focus cameras, the most basic of all cameras, have a nonadjustable lens.
Most models have a single aperture setting and only one or two shutter speeds. In general, a fixed-focus camera can take satisfactory photographs in ordinary daylight but not in dim light, because its aperture does not admit much light. Many fixed-focus cameras can take flash pictures. Disposable or recyclable cameras are a kind of fixed-focus camera that combines a plastic lens, a shutter, and film in one small box. The entire camera is brought to the photo lab when the roll of film has been exposed.