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How Lenses Work

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How the Eye Controls Autofocus

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.

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