Autofocus via human vision - Focus with photographer's eye
Autofocus via human vision - Focus with photographer's eye | ||
|
Few things are as easy as simply looking. You turn your eye, and everything else happens automatically. Muscles tug on the cornea to pull it into the proper shape to bring into focus whatever you're looking at. Other muscles contract or relax their holds on the iris so that the pupil shrinks in bright light or expands in dim light so the light-sensitive cells lining the retina at the back of your eyeball see details without strain. If only other things, such as focusing a camera, were so easy. If the true object of your photo isn't dead-center in your viewfinder, most cameras—digital or film— require you to do a sleight of hand with the shutter button, aiming at where you want it focused and then pressing the button halfway while you frame the picture for real. But some cameras, pioneered in the film days by Canon, have found a way to make focusing, literally, as simple as looking. When the photographer puts his eye to the viewfinder, he sees an image that has come through the camera lens and been reflected up to a focusing screen, a plate of glass that has been ground to have a rough surface on one side. The rough surface catches the light so it can be seen, like the image on a rear-screen projection TV. The photographer sees the image on the screen after it has been reflected up by a mirror and passed through a prism, which flips the reversed image from the screen 180° so the photographer sees the image in its proper orientation. A smaller mirror behind the main mirror sends the image to the autofocus sensor in the base of the camera. At the same time, an infrared LED (light emitting diode) shines light, invisible to the photographer, on his eye. The infrared light reflects off the eye. It passes, with no noticeable distortion, through a lens used to focus the photographer's vision on the reflected image from the ground glass. Then the infrared light reflects off a dichroic mirror set at an angle. A mirror has a thin coating of transparent metal oxides chosen, in this case, to reflect infrared light while letting visible light pass through the mirror. The reflected light passes through a lens that focuses the image of the photographer's eye on a complementary metal-oxide semiconductor (CMOS) autofocus sensor. This is an array of CMOS photodiodes that creates a current when they are stimulated by the infrared light. The sensor detects the image of the eyeball, iris, and pupil and sends information about the location of the eye's image on the sensor to a microprocessor. Earlier, to calibrate the mechanism to the physiology of the picture taker's eye, the photographer had looked through the viewfinder in various directions. The processor stored the images that the eye's movement made on the processor during the calibration. Now it compares the new information from the sensor with the stored data. From that comparison, the microprocessor quickly determines what part of the image the photographer is looking at. It conveys that information to the camera's focusing mechanism, which is capable of evaluating up to 21 metering zones that are linked to 45 autofocus points that cover the whole frame of the picture. The signals tell the autofocus which of the zones to pay attention to, and the autofocus sends signals accordingly to motors that adjust the lens to focus on the target of the photographer's eye. | ||