Why some images are not focused?
Why some images are not focused? | ||
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You can more easily understand focus if you see why some images are unfocused. Picture focused rays of light—the ones converging on a point of the focal plane—as a cone. If there's nothing at the focal plane to stop them, such as film or a digital image sensor, the light continues on to form another cone that's the mirror image of the first. The location of a focal plane is partly the result of the optical characteristics of the lens and of the distance from the lens to the subject. Objects at different distances, some too close to the lens and some too far away, will be out of focus. The tips of the two cones meet at the same point on the focal plane. If an image sensor were located either in front or in back of the focal plane, the light would form not a point of light, but a circle. It would be unfocused. Because the rays of light would not be concentrated, the circle would be hazy, and dimmer than a focused point of light. The unfocused parts of the image, particularly when they contribute to an artistic effect, are called the bokeh, a great word for Scrabble and outpretending the pretentious at parties. Some cameras have fixed focus lenses that don't require adjustments for each shot. This type of lens tries to get everything in focus from a few feet in front of the camera to infinity, but it's a poor compromise. Nothing is in truly sharp focus, and you can forget about close-ups. Rather than bring an object into focus by walking toward or away from it, the photographer turns a focusing ring that runs around the barrel of the lens. (Virtually all digital cameras also provide autofocus, which adjusts the lens without the aid of the photographer. That's the subject of the next two illustrations.) Attached to the ring is a part of the lens's barrel with a spiral groove cut into it. As the groove turns with the focusing ring, it moves a pin from another part of the barrel. The groove-pin arrangement translates the focus ring's circular motion into linear motion. That motion, in turn, moves the lens elements toward and away from one another. The different distances among them change the optical effect they have on each other and shift the paths that light beams take as they careen through the lenses. When the light rays reach the digital image sensor, the optical changes now bring to focus a part of the scene that's closer or farther away. | ||