The naked eye has the capability of seeing a distance object light-years away under the right conditions. The eye’s line of sight, the brightness of light from the object, and eye health are factors that determine how far the human eye sees. The eye’s clarity and sharpness are measured during an eye exam and can be affected by any ocular pathology that affects eye health or uncorrected refractive error.
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- The object’s brightness and an unobstructed view determines the distance from which the human eye is able to see the object.
- Visual acuity is the clarity measure of one’s vision.
- Eye health, refractive error, and ocular aberrations and components affect vision.
How Far Can The Human Eye See?
Viewing the night sky, a full moon or a planet like Saturn through a telescope is different than viewing with the naked eye. Exactly how far can the human eye see? Several factors affect the distance you can see including the capability of the human eye, the brightness of the object and an unobstructed view.
M33 is a spiral galaxy located within the Triangulum galaxy. It has a brightness magnitude of 5.7, which makes it one of the most distant objects that can be viewed by the unaided eye (under exceptionally clear and dark skies). M33 is slightly further away from us than the Andromeda galaxy which is located 2.6 million light-years from earth.1,2
Due to the curvature of the earth, the farthest object one can see is about 3 miles, but if the earth was flat or one was at a higher vantage point like on top of a mountain, it would be possible to see bright lights in the distance hundreds of miles away, according to livescience.1
Despite 70 years of investigations, the limits of human vision still remained unclear. Rod cells respond to individual photons, yet it remains unknown whether a single-photon incident on the eye can be perceived by a human.3 Research has demonstrated the number of photons at the cornea that reach the retina is somewhere between 10-30%.4
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Visual Acuity and The Human Eye
When your vision is checked by an eye doctor or optician, 20/20 is considered normal vision. Visual acuity is a measure of the clarity or sharpness of one’s vision. Visual acuity is a measurement of your ability to see a distance object or a near object and taken using a Snellen chart. The measurement of vision is based on what a “normal” person can see at a distance of 20 feet. For example, 20/20 vision means the patient can see the same as what a normal person sees at 20 feet. Visual acuity is the smallest retinal image appreciated by the eye.5
Your eye health, refractive error, spatial distance between the photoreceptors, optical aberrations, and pupil size can affect your visual acuity.
Eye health can affect your visual acuity. Any ocular pathology or disease process that affects an ocular structure can limit human vision. Conditions of the cornea like keratoconus cause aberrations in vision and even distort the patient’s view. Similarly, cataracts can cause obstructions in your central line of sight and distort colors. Diabetic retinopathy and age-related macular degeneration are eye conditions that make it difficult to see.
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If one cannot view a distance object than they are considered myopic (near-sighted). Individuals with myopia have eyes with a slightly longer length than normal. However, if it is difficult to see near objects, patients are hyperopic (far-sighted) with shorter eyes than normal.7
Lastly, astigmatism is one of the most common refractive errors and occurs when parallel rays of light entering the eye are brought to a focus at two distinct focal lines perpendicular to each other, rather than to a single focal point resulting in halos and glare.8
The Human Eye: How It Works
The cornea, iris, and lens compose the front of the eye. The retina and optic nerve, equally important structures are located in the back of the eye.
Front of the Eyeball
The cornea is the clear dome over the surface of the eye. Its purpose is to protect the eye and refract rays of light. The structure of the eye called the iris allows the appropriate amount of light to enter the retina. The iris is like the shutter of a camera. It allows enough light to enter the eye to activate the retina. The iris dilates in the dark and constricts in the light allowing more bright light through the pupil under scotopic (dark) conditions than under photopic conditions (light). For this reason, when driving, it may be more difficult to see a distant object on a dark night compared to a clear day. Made of tissue, the crystalline lens can change its shape to adjust the power of the eye and to focus the image on the retina called accommodation.9
Back of the Eyeball
The retina is a layer of photoreceptors and glial cells that captures incoming photons and transmits them along neuronal pathways as both electrical and chemical signals to the brain in order to see a picture.10
It is composed of two different types of cells: rod cells and cone cells. The activation of rod cells and cone cells, also called photoreceptors, initiates our vision. One type of rod cell exists for dim light vision and three types of cone cells enable one to see colors.10 Finally, the signals from the eyeball are sent to the brain through the optic nerve.
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Putting It All Together
The capability of human vision remains unknown though a bright object and an unobstructed view are important to see a distance object like the Andromeda galaxy. Additionally, the visual acuity of the human eye is affected by eye health and refractive error. Types of ocular pathology like age-related macular degeneration of astigmatism can have an affect on how the eye sees. Furthermore, visiting an eye doctor to have your vision checked, and your glasses prescription updated is vital to optimizing the capabilities of the eye.
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