You may not spend a lot of time staring off toward the horizon, trying to make out distant objects. But if you do look into the distance at ground level and have an unobstructed view, how far can the human eye see?
Pretty far, as it turns out.
ContentsIf you're a person with normal vision acuity — a rating of 20/20 — and you gaze horizontally from around 5 feet (152.4 centimeters) above the ground, you can see about 3 miles (5 kilometers) into the distance, which is the point at which the Earth's curvature bends away so that the surface is no longer in view.
And while your ability to discern objects depends upon their size and the how much light the distant object emits, on a dark night it's possible to see a candle flame from about a 1.5 miles (2.4 kilometers) away, according to Dr. Eric Lowell Singman. He's a practicing ophthalmologist with the University of Maryland Medical System, who also is a professor of ophthalmology with the University of Maryland School of Medicine.
To understand how our eyes can see a distant object like a candle flame flickering, it helps to know something about normal vision and how the human eye works. While visual acuity varies among individuals, the naked eye definitely is a pretty impressive piece of equipment.
It's capable of 12 times the resolution of a high-end smartphone's camera. As Singman notes, we can see a nearly infinite range of colors and shapes, as well as detect very small changes in brightness and minute amounts of motion.
"The eye turns light into biometric energy transmitted to the brain," Singman explains. But that's just the high-level view. Human vision is a complex, intricate process. Light reflects off objects and passes through the cornea, which bends light rays. and the lens
When enough light passes through and reaches the retina. Then a layer of tiny nerve cells lining the back portion of the eye known as rods and cones that help send electrical impulses through the optic nerve to your brain, which creates an image or visual perception.
"Some see colors, some see black and white," Singman says. "Some see motion only in one direction, others see motion in another direction. Some see shapes. Some just look at the overall ambient lighting."
Together, those cells work as a team to tell your brain what to visualize, transmitting information about what time of day it is, what's moving and what colors are visible. "These are actually different separate channels," Singman says. "And then your brain takes the images and converts them into something that your conscious mind can appreciate."
In addition, the human visual system also transmits and creates visual information that you aren't even necessarily aware of — for example, the amount of ambient light, which controls your natural rhythms of day and night.
As Singman previously explained, without obstructions in the way, you can see about 3 miles (4.8 kilometers) into the distance from ground level, before you reach the point where Earth curves enough to block your view of what's on the ground. That's based on an imaginary person who is 5 feet (1.5 meters) tall.
"If I was 6 feet [1.8 meters] tall, I could see farther," Singman says. The higher you go in elevation, the farther into the distance you can see. A person looking out from the observation deck of Burj Khalifa, a 2,716.5-foot (828-meter) skyscraper in Dubai that is the world's tallest building, reportedly can see for 60 miles (96.5 kilometers), according to a 2010 New York Times article.
And our visual acuity extends beyond the horizon even. If the curvature of the Earth didn't exist and instead Earth was a flat surface, we'd have an uninterrupted visual angle from our eyes to the farthest object visible.
The atmosphere also has a major factor in how well you can distant objects, according to Dr. Timothy McCulley, a professor and chair in the department of ophthalmology and visual science with McGovern Medical School at UTHealth Houston.
Theoretically, in a vacuum there's no limit to how far away your eyes could see since light rays can travel an infinite distance, McCulley says. But on Earth's surface, "you're seeing through the atmosphere, and yellow particular matter in the air is scattering the light," he says. "So depending on the medium that the light is traveling through, those electromagnetic waves will or will not reach your eyes."
A similar problem occurs on a foggy day or night. "The light just doesn't get through," McCulley explains. That's why we're able to see that candle flame flickering when there's a dark night sky, but not when it's cloudy out.
Even though we can see a glowing object far in the distance, the human eye has its limitations, especially when it comes to visual acuity, the scientific-medical term for sharpness of vision.
The cone cells — photoreceptors that are sensitive to red, green and blue wavelengths of light — are concentrated in the macula, an area in the middle of the retina. Our sharpest vision is at the center of the macula, in a spot called the fovea that can zero in on a small part of the world in front of you.
"You might think that you have a very wide visual field, but the reality is that your area of clearest vision is actually just a few degrees. You don't see the whole world clearly," Singman says. "If you take a big letter E, big as your hand, and move it about 15 to 20 degrees away to the side, you wouldn't be able to tell what letter it was. The clarity of your vision drops pretty quickly, once you get off the center."
That's the reason why macular degeneration, an eye condition in which the macula becomes damaged so that you lose that central vision, can be such a serious problem for people who get it. (The risk increases as you get older, according to the National Eye Institute.)
"You've got to remember that the brain is a really big part of this process," Singman says. That's evident in people who have some sort of brain injury that interferes with their vision, even if there's nothing wrong with their eyes.
"There are types of brain damage where you can't see something move, or where you can't recognize faces — you'll look and pick out an eye or a nose or mouth, instead of the whole face," Singman says.
The brain, in fact, can perform tricks to make up for the eyes' shortcomings. Singman recalls the case of a patient who after eye surgery, suddenly discovered that when he covered that eye, he couldn't see out of the other, supposedly good eye.
When a doctor examined the patient, it was discovered that he had a cataract, a natural clouding of the lens on that eye, but apparently had never noticed it because of his brain's ability to filter out the blur. "That's a classic trick — the brain can shut things off," Singman says.
Now That's InterestingWhat's the farthest the human eye can see if it's unobstructed? Look up — to space that is. On a dark night sky without light pollution, you should be able to see the Andromeda galaxy, which is 2.5 million light years away. That's the equivalent of 15 quintillion miles or 24 quintillion kilometers.
