Automatic Focus Mechanisms (Autofocus): Digital Camera Focus

If we capture sun rays with a magnifying glass or lens, and we make them converge on a plane (a focal plane or a translucent sheet of paper), what we obtain is an image of the sun that is at a distance from the lens equal to the focal length. Any other relative positions provides a blurry image and of greater diameter ( confusion circle ). On the other hand, if we capture the rays of light which come from a body placed at a finite distance (for example, the filament of a light bulb), we can observe that the image is formed at a greater distance from the lens than the focal distance. The shorter the distance of the object to the lens, the farther away from it, it's image is formed. Every focal distance (from the object to the lens) requires a different relative position from the lens to the sharp focal plane of the image. The position of the planes of sharp focus correspond to the distances respond to the following relation:

1/ p + 1/ q = 1/ f

• were p = the distance from the point to the lens (focal distance);
• q = the distance between the lens and the image of the point;
• f = focal length.

For example, when p is infinity, the first term becomes equal to 0 and we have the expression 1/ q = 1/ f , which implies that q = f : when the object is at infinity, it's image is formed at a distance from the lens that is equal to a -or we have defined- the focal length; also, this law implies that there is a limit of the focal distance, precisely when p = f , since in this case q becomes infinity. We have seen that the focus mechanism permit's the displacement of the lenses to the adequate distance for the film or digital sensor according to the focal length. The need for exactitude in the focus derives precisely from the fact that to each focal distance, there corresponds, theoretically, one and only one position of the focus ring.

Freeing the user from the need to precisely focus has been an eternal aspiration for camera manufacturers. Until the coming of electronics in camera design, the more economical models didn't have a focus mechanism, having a lens with great depth of field and a fixed focus. Even functioning acceptably for distances above 2 m, this solution implies a notable loss of quality in the results. Thanks to new technology, authentic automatic focus mechanisms are being developed to be more precise every time.

•  Telemeter systems : In a direct viewfinder camera, the sharpness can't be seen through the viewfinder except through indirect mediums (double image). An automatic focus system capable of calculating the effective distance of the camera to the point we want to focus on substitutes the users eye. Once the distance is found, the value is transmitted to a servomotor which displaces the lenses.

Ultrasound autofocus ( sonar ) measures the time passed between the emission of an impulse and the reception of its eco , returned after bouncing on the surface we wish to focus on; through the measured time the distance can be calculated.

Infrared autofocus emits a series of impulses with variable angles along the axis. For a determined angle the eco receptor picks up the signal returned by the subject, which allows the system to calculate the distance it's found. These systems are based on the calculating of the distance to a determined object; nonetheless, if a mistake is made in the value that the system assumes as a focus distance, the result would be a blurry image. This can happen quite easily: when the distance is short and the scene has a certain depth; when the impulse is absorbed or bounced in another direction; when the focal distance corresponds to object interposed between the camera and the subject; in direct viewfinder cameras, it's almost impossible to detect.

•  Contrast Systems: In a manual focus on a screen, what makes us decide on a determined position of the ring is the sharpness of the image; we rarely take in account the distance scale, since what really matters is the sharpness of what we want to photograph. How can we define sharpness so that an automatic focus system can calculate it? An out of focus image is a mix of confusion circles -blurry aspect; the border between two zones of different brightness are drawn-; on the other hand, when an image is focused in strict sense, the border between two parts of the scene is more drastic than in any other position of the focus ring (maximum local contrast). Contrast systems are incorporated in digital and reflex cameras an make the focus in a small zone of the frame. Once focus has been made, we can reframe as we wish and shoot.