A $4,000 iPhone gadget replaces a 30-pound, $20,000 machine, allowing eye doctors to visit patients they couldn't reach before.Anyone who's had an eye exam in the past few decades has stared into an autorefractor—the device, the size of microwave oven and costing up to $20,000, that bounces an infrared beam off the retina to determine the correction needed for an image to come into focus.