What if you could see like an eagle?

Look at something far away. Can you see the tiny parts?

An eagle could see them much better! What if you had eagle eyes?

Look out of a window at something far away, like a tree or a sign. The farther away it is, the harder it is to see the small details, like single leaves or letters.

Eagles are famous for their sharp eyes. Cornell's All About Birds describes golden eagles soaring on steady wings or diving after jackrabbits and other small mammals. So what if you could swap eyes with an eagle? What could you see that you can't see now? And is there anything an eagle can't see that you can?

Visual acuity, the ability to resolve fine detail, is measured in cycles per degree: how many pairs of light and dark stripes an eye can tell apart within one degree of its view. Diurnal birds of prey hold the highest values ever measured.

This thought experiment gives a human an eagle's eyes and asks what would change. It follows three threads: how sharp eagle eyes really are, how their anatomy achieves it, and what the evidence says about ultraviolet vision in eagles. Some claims are well measured; others rest on single animals or are open questions.

Light goes into your eye. At the back is a special layer called the retina.

Tiny cells there turn the light into signals for your brain. Then you see!

The National Eye Institute explains how your eyes work. Light passes through the clear front of your eye and the lens, which focus it onto the retina, a light-sensitive layer at the back. Special cells there, called photoreceptors, turn the light into electrical signals that travel to your brain.

You have two kinds of photoreceptor. Rods work in dim light. Cones work in bright light and let you see colors. In the middle of your retina is a small spot called the fovea, packed with cones. When you look straight at something, its picture lands there, so that's where you see the sharpest detail.

In the human eye, the cornea and lens focus light onto the retina, where photoreceptors convert it into electrical signals carried by the optic nerve to the brain (National Eye Institute). Rods are highly sensitive and serve dim-light and peripheral vision; cones need brighter light and mediate color through three types, peaking near 420, 530 and 560 nm (OpenStax).

The fovea is the retinal region of highest cone density and sharpest vision; when we fixate an object, the eyes aim its image there. Human foveae are shallow, and each eye has one. Resolution depends on how finely the image is sampled, which is set by eye size and the spacing of cones.

Now pretend you have eagle eyes. Faraway things look much sharper!

You might spot a tiny bug across the playground.

Scientists have tested how well birds of prey see by training them to pick out fine stripes. Wedge-tailed eagles can see about twice as much fine detail as people can.

With eyes like that, you might be able to read a sign from about twice as far away, or spot a small animal in a field far below a mountain. That part is our idea about what twice-as-sharp eyes might mean. Eagles use that sharp sight to hunt, and for them, vision is probably their most important sense.

Behavioral studies found that large raptors such as Old World vultures and wedge-tailed eagles (Aquila audax) have about twice the spatial resolution of humans (as summarized by Mitkus and colleagues, 2017). Potier and colleagues (2018) cite 142 cycles per degree for the wedge-tailed eagle. A 2017 review describes the peregrine falcon's acuity as about 140 cycles per degree, with some eagles similar.

If acuity doubled, a detail that a person can just resolve at one distance might be resolved at about twice that distance; that is a geometric idea, not a measurement. The same papers stress that most work rests on one animal or a few, with varied methods.

Some birds of prey have eyes as big as yours, or bigger!

Their eyes are packed with lots and lots of tiny light cells. More cells make a sharper picture!

Eagle-type eyes are big. Some birds of prey have eyes as big as a person's, or bigger, even though their bodies are much smaller. A big eye makes a big picture on the retina, and eagles pack that picture with many tightly squeezed light-sensing cells.

Many birds of prey also have two foveas in each eye, two extra-sharp spots, where we have one. A deep one looks out to the side and a shallower one looks forward. Scientists still don't know exactly what each one is for.

Mitkus and colleagues explain that some raptors have eyes as large as or larger than human eyes, giving a long focal length and a large retinal image. In the wedge-tailed eagle's deep fovea, cones sit about 1.6 µm apart, center to center. A 2017 review adds the tubular eye shape and large pupil, and notes that the vitreous in the deep fovea could act as a third lens, adding magnification.

Unlike the single shallow human fovea, many raptors have two: a deep central fovea viewing the lateral field and a shallower temporal fovea viewing the frontal field. The deep fovea has the higher acuity, but the exact role of each is unknown. Sharp eyes are not best at everything: Potier and colleagues found Harris's hawks about ten times less sensitive to contrast than humans.

Many birds can see a kind of light we can't see. It is called ultraviolet.

But eagles probably can't see it very well. Even eagle eyes have limits!

Many birds can see ultraviolet light, a color of light that is invisible to people. You might expect eagles to see it too. But when scientists read the genes of a golden eagle, they found genes for seeing violet light, not ultraviolet.

Some scientists once thought certain birds of prey might follow ultraviolet clues left by voles, small animals they hunt. Other scientists later argued that the clues probably would not stand out. So with eagle eyes, you'd see sharper detail, but probably not a hidden ultraviolet world. Scientists are still studying it.

Hunt, McClure and Allison (Journal of Raptor Research, 2015) summarize the evidence. Many birds are sensitive to ultraviolet light, a trait set by the SWS1 pigment in one of their four single-cone types. Genetic studies suggested raptors generally lack UV sensitivity, and the golden eagle (Aquila chrysaetos) genome revealed genes for violet rather than near-ultraviolet sensitivity. Mitkus and colleagues give a peak of 405 nm for this cone in the common buzzard.

Field observations had suggested that some raptors use the UV reflectance of vole urine when hunting; as reported by Hunt and colleagues, Lind and colleagues (2013) maintained that the difference from the background was likely indistinguishable to raptors. Eagle-sharp eyes, then, may come without UV vision.