Skip to main content

How Far and Close Can We See?

I'm a writer with 6 years of experience in writing from Venezuela, South America. I love learning about science and discoveries

If you stand anywhere on the earth on flat ground and glance at the horizon, your sight will be limited by the curvature of the earth. That curvature makes your view have a limit of five kilometers if the earth were flat ... if it were because it is not...

Your visual range would go much further on a flat horizon. If you were on top of a mountain, you could see lights that are thousands of miles away. Even on a dark night, you would be able to visualize a candlelight 30 miles away.

Human vision affected by the curvature of the Earth

Human vision affected by the curvature of the Earth

A light of a candle could be noticed 30 km away on a flat Earth

A light of a candle could be noticed 30 km away on a flat Earth

According to studies, how far we can see depends on the light particles that are emitted from an object or that are reflected from that object, which can be captured by our eyes. The farthest object that is visible from here is the Andromeda galaxy which is 2.6 million light-years from Earth.

The trillion stars in this galaxy emit enough light for a few hundred photons to hit Earth every second and be captured by our eyes under the right conditions. Of course, your eyesight won't do the same on a rainy day or if it's cloudy or Snorlax is in the middle. But the most predominant factor will always be visual acuity.

Andromeda galaxy from Earth

Andromeda galaxy from Earth

Visual acuity

If you have ever been told you have 20/20 vision, maybe you heard it from somebody who doesn't know what 20/20 means.

20/20 is a scoring system for determining how much we see. Herman Snellen, a Dutch ophthalmologist, was the creator of this system focused on studying the visual acuity of his patients.

Eye chart

Eye chart

Visual acuity is the maximum capacity that our eyes have to distinguish an object at a certain distance. Based on this, Snellen defined the size of the smallest letter that the human eye can perceive.

The 20/20 numerator means the distance for which the test was designed (20 feet or 6 meters). The denominator (20/20) refers to the distance at which our eyes should read that font size. As the denominator increases, visual acuity is lower.

If a person has a 20/20 vision (he can see 20/20 size letters), it means that he has good visual acuity. Well, you are reading a letter at 20 feet whose size must be read at least 20 feet. On the other hand, if a person can read clearly up to 20/40 letters, he is reading at 20 feet a letter that should be read at 40 feet. In other words, he can’t see very well.

Snellen charts, also known as visual acuity optotypes, consist of a series of large and small letters. They generally start at 20/400 or 20/200 and end at 20/20 or even reach better visual acuities such as 20/15 or 20/10.

Hawks, owls, and other birds of prey have much higher visual acuity than humans. Animals like hawks have much smaller eyes than humans but have quite a few sensors or cones clustered in a small space, giving them a vision that can be up to eight times sharper than humans. A hawk can have 20/2 vision.

Some people who have very low visual acuity cannot go out on a sunny day without wearing glasses, which helps them improve contrast and decrease glare.

An athlete with a vision greater than 20/20 would struggle, as this would affect his performance. With that visual acuity, he would lack certain dynamic visual abilities that would allow him to react to moving objects as soon as other athletes. So normal is considered 20/20, but it doesn't mean perfect.

According to this, if you had a visual acuity of 20/20, off the ground, you could notice the light of a candle 30 miles away if the weather was fair.

View of a candle 48km away with a 20/20 vision on a flat earth

View of a candle 48km away with a 20/20 vision on a flat earth

Now that's from afar, but how about close? What is the smallest thing we can see? What is the limit of our vision? Can we see a flea? The tip of a hair? A smurf?

Smaller objects

According to some calculations, using a comfortable distance for your eyes not farther or closer than 25 centimeters from the object you want to see, the limit for you to see something with the naked eye is 0.1 mm in diameter. That's roughly the size of the largest known bacteria, Thiomargarita namibiensis.

Thiomargarita namibiensis, near a fruit fly

Thiomargarita namibiensis, near a fruit fly

The size of this bacteria ranges from 0.1 to 0.3 mm, and it's that little white thing that seems to glow on the side of fruit flies, which is quite surprising because we normally think bacteria are microscopic. As a reference, the size of a fruit fly varies between 3 and 4 millimeters. So that bacteria is the smallest thing we can see with the naked eye.

However, there are objects like spider webs that have a diameter less than 0.1 mm, and we can still see them. A spider web is 0.003 millimeters in diameter but we can see it several meters away. How is it possible?

Spider web

Spider web

This is because it reflects light and it is long. It reflects enough light for us to see it. Light is ultimately what we need to see.

Optical microscopes

We can also use light to see much smaller objects that our eyes cannot see using optical microscopes.

Optical microscope

Optical microscope

Objects get harder to see when they are tinier than the wavelengths. This is how some objects look under an optical microscope.

The limit to seeing something with an optical microscope is 200 nanometers. Beyond that size, objects become too small to be seen in visible light. Certainly, there are other methods to identify these objects. We need other methods like electron microscopes that use wavelengths.

Electron microscopes

Electron microscopes allow visualizing objects that are very small, and objects that are part of our daily life but often go unnoticed, such as a round-tipped pen, salt, a needle, a thread, the hair of a mustache, or mustache hairs.

A round-tipped pen under an electron microscope

A round-tipped pen under an electron microscope

Scientists also use such electron microscopes to view animals. This is how everyday objects and animals look under an electron microscope.

Related Articles