A bird’s eye view: How does human eyesight compare to an eagle’s?
Humans and birds of prey—like eagles—have something in common: they’re both creatures that rely on eyesight in their daily lives. While humans have adapted to depend on our other senses, our strong eyesight has helped us in a big way, evolutionarily-speaking. In this blog post, we examine the strength of eagle eyes, pitting man versus beast, establishing how human eyesight measures up to eagles’.
What does visual acuity mean?
Visual acuity is the term used to describe the eye’s ability to make out sharp, crisp details without any blurriness. There are many factors that contribute to an eye’s visual acuity: size, dimensions of the parts inside the eye, and the number of receptors located inside the retina are all factors that strengthen visual acuity.
20/20 vision, explained
20/20 is a common term that describes great vision. Essentially, this term is a measurement of visual acuity. A person’s visual acuity is measured by whether they can read a Snellen eye chart (the measurement system many optometrists use to test vision, a chart featuring rows of letters at varying sizes). When a person has 20/20 vision, they can see a small row of text clearly from 20 feet (approximately six metres). The best recorded vision in humans is 20/10 vision—the ability to see objects clearly from 20 feet when a normal human can only see them at 10 feet. The clearest vision a human can obtain is through the science of a vision correction procedure. A surgery like LASIK can give a person 20/20 vision. Many patients have even reportedly achieved 20/15 vision following laser vision correction, which is impressive, especially if you’ve had to wear glasses for most of your life.
Most animals don’t come close to the 20/20 visual range. Dogs or cats, for example, can only see clearly from 20 feet while a human with otherwise normal vision can see from approximately 100 feet. Don’t worry, we didn’t ask your dog Spot to bark out the letters on the Snellen eye chart—animal vision is measured by other tests.
But no one species even comes close to eagles and other birds of prey. These types of birds have such strong vision because their retinas are packed with light-detecting cells called cones. They also have a much deeper fovea, a cone-rich structure in the back of the eye that acts like a telephoto lens (a long-focus lens) on a camera. Having these traits means that eagles have a visual acuity of 20/5 or 20/4.
Eagles have high-definition vision
When compared to other creatures, human eyesight does see bright, vivid colour crisply and clearly. But this doesn’t hold a candle to how eagles perceive the world. Many birds of prey can see colours on an even wider spectrum than humans do. Their eyes pick up more shades and contrasts. They can even see ultraviolet (UV) light. The ability to make out UV light helps these birds spot traces left by prey—urine or fur, for example. This make their prey stand out against the uniform colour of a field.
Eyes in the back of their head
Human eyes are positioned on the front of our head. This gives us binocular vision, so except for the images on the periphery of our vision, we use both eyes to see clearly. This gives us the ability to see how far an object is (a phenomenon known as depth of field) and how quickly something is moving (a speeding car, for example). The downside to our eye placement, however, is our poor peripheral vision: humans can only see 180-degrees. Eagles, however, are predatory birds, and their eyes are located towards the front of their head. However, their eyes are angled at 30 degrees away from the midline of the face, so they have a 340-degree field of vision. They have better peripheral vision than an owl, but not quite as good as that of a woodcock, which has an impressive 360-degree field of view. Still, they’ve got human eyes topped.
Focus on near and far
For evolutionary purposes, the human eye is designed to focus on objects. Tiny muscles in our eyes help us see things up close so that we can focus on objects just a few inches from our face—this process is known as accommodation. Unfortunately, as all humans age, these muscles become less and less effective, which is why so many of us need reading glasses (a condition known as presbyopia).
Eagles have us beat in this area as well. Not only do they have a lens that changes shape (just like us), their cornea—the clear, outer portion of the eye—also can change shape to better focus on near and far objects.
Eye conditions in animals
While animals may not experience eye conditions like astigmatism, myopia, or hyperopia to the same degree as humans, it is however possible for many animals—including eagles—to be diagnosed with cataracts (a clouding of the eye’s natural lens).
Eagle eyes of your very own
What we can conclude is that birds of prey have superior vision to humans. But, human beings also have strong vision—and if their eyesight is less than desired, corrective procedures, like LASIK, can help. One quick, 10-minute procedure has the potential to transform your blurry vision into 20/15 clarity. So while you may not need to hunt your prey from mile-high distances, you’ll be able to do things like see road signs on your next road trip, water-ski without worrying about wearing contact lenses, and live your life freely—just like the soaring eagle.