If you swapped your eyes for an eagle's, you could see an ant crawling on the ground from the roof of a 10-story building. You could make out the expressions on basketball players' faces from the worst seats in the arena. Objects directly in your line of sight would appear magnified, and everything would be brilliantly colored, rendered in an inconceivable array of shades.

The more scientists learn about eagle vision, the more awesome it sounds. Thanks to developing technologies, some aspects of their eyesight may eventually be achievable for humans. Others, we can only imagine.

Eagle eyes

Eagles and other birds of prey can see four to five times farther than the average human can, meaning they have 20/5 or 20/4 vision under ideal viewing conditions. Scientists have to cook up special experiments to judge eagles' eyesight — your optometrist's alphabet eye charts are of no use, after all — and one common setup involves training the birds to fly down a long tunnel toward two TV screens. One screen displays a striped pattern, and the birds get a treat when they land on it. Scientists test their acuity by varying the width of the stripes and determining from what distance the eagles begin to veer in the correct direction.

According to William Hodos, a distinguished professor emeritus at the University of Maryland who has studied the visual acuity of birds since the 1970s, two eyeball features confer eagles' sharper vision. First, their retinas are more densely coated with light-detecting cells called cones than human retinas, enhancing their power to resolve fine details just as higher pixel density increases the resolving power of cameras.

Second, they have a much deeper fovea, a cone-rich structure in the backs of the eyes of both humans and eagles that detects light from the center of our visual field. "Our fovea is a little shell or bowl, while in hawk or eagle it's a convex pit. Some investigators think this deep fovea allows their eyes to act like a telephoto lens, giving them extra magnification in the center of their field of view," Hodos told Life's Little Mysteries.

On top of sharp focus and a central magnifier, eagles, like all birds, also have superior color vision. They see colors as more vivid than we do, can discriminate between more shades, and can also see ultraviolet light — an ability that evolved to help them detect the UV-reflecting urine trails of small prey. But there's no way to know what these extra colors, including ultraviolet, look like. "Suppose you wanted to describe the color of a tomato to someone who was born blind. You couldn't do it. We can't even guess what they're subjective sensation of ultraviolet light is," Hodos said. [Red-Green & Blue-Yellow: The Stunning Colors You Can't See]

Life with 20/5 vision

Eagle vision wouldn't change how we perform most daily activities — such as reading computer screens or the newspaper, or finding milk in a crowded refrigerator — but how we perceive the world and use our eyes would certainly be different. It's perhaps easiest to consider our new powers in the context of how eagles use them: for hunting.

On top of the ability to see farther and perceive more colors, we would also have nearly double the field of view. With our eyes angled 30 degrees away from the midline of our faces like an eagle's, we would see almost all the way behind our heads with a 340-degree visual field (compared to normal humans' 180 degree field); this would confer a clear advantage in hunting and self-defense.

With eagle eyes, we would swivel our heads constantly. To locate prey or any other object of interest in the distance, you'd periodically turn your head to the side to sweep your fovea (telephoto lens) across your field of view. After spotting what you're looking for in this manner, you'd redirect your head toward it and use stereoscopic vision — combining the viewpoints of both eyes to gauge distance — to calibrate the speed of your approach.

Enhanced perception and hunting prowess would likely come with a few drawbacks. "I would say that birds probably have a greater proportion of their brain volume devoted to visual processing than other groups of animals. Now the question of what it comes at the expense of: most birds appear not to have a well-developed sense of smell or taste," Hodos said.

It's more difficult to say how your more sophisticated cognitive processes would fare. "Birds have areas that seem to function like the cortex [the part of our brains responsible for memory, language and complex thought], but it's arguable. But in terms of their ability to solve problems and so on, they match what many mammals can do. Many birds have superb memory," he said. [The 5 Smartest Non-Primates on the Planet]

Maximizing our potential

Eagles' high-flying lifestyle requires better vision than humans need, and the physical properties of our eyeballs limit us to 20/10 or 20/8 vision at best. Natural vision that good is extremely rare, but research by David Williams, director of the Center for Visual Science at the University of Rochester, and his colleagues may soon enable laser eye surgeons to achieve 20/10-or-better vision for a large percentage of patients, placing their visual acuity halfway between that of humans and eagles.

Williams and his colleagues use an instrument called a wavefront sensor to detect distortions in human vision. They shoot light into the eye and observe how it bounces back through hundreds of tiny lenses in the sensor. The aberrations in patterns created by those lenses serve as a map of the eye's mistakes. Customized surgical techniques are being developed to implement the results of patients' wavefront measurements, in order to correct their vision beyond 20/20.

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