The Eye

How it works

The human eye is an incredibly sensitive and complex sensory organ. Our eyes allow us to gather information about the world around us and experience its beauty and colors. In fact, the human eye can distinguish about 10 million different colors.

In many ways, our eyes work like a camera: by focusing incoming light onto the back of the eye—known as the retina—our eyes give us the ability to see. From the retina, millions of receptors send the information to the optic nerve, which relays it to the brain, producing a single, three-dimensional image. To create a clearly focused image, all parts of the eye need to work together perfectly.

Vision can be impaired due to an irregularity in the shape of the eye (called a “refractive error”) or by the natural aging process. This may affect quality of life and can be very inconvenient and frustrating. The most common sight issue is nearsightedness, also known as myopia.

The anatomy of the eye

Emmetropia: perfectly focused vision
The science of sight is highly intricate. In simple terms, with emmetropia or normal vision, light emitted or reflected from objects travels through the parts of your eye to form a clear and sharp image.

Parts of the eye
The eye is a complex structure, with all the different parts working together to create an image. When we look at an object, the incoming visual information passes through the parts of the eye and is converted into sight by the brain. Anatomically, the eye can be divided into three parts: front, inner and back.

Front of the eye

1. Sclera: The white outer area that forms and supports the structure of the eye.

2. Cornea: The clear front part of the eye that bends light as it enters the eye.

3. Pupil: The opening in the center of the iris, where incoming light passes through.

4. Iris: The colored, ring-shaped membrane behind the cornea that controls the size of the pupil, which in turn regulates the amount of light entering the eye.

5. Conjunctiva: The clear covering of the sclera that helps keep the outer eye healthy.

Inner eye

6. Natural Lens: The clear structure in the eye behind the iris, the lens helps focus light onto the retina. The lens is able to change its shape so the eye can focus on objects at various distances. This focusing ability of the lens is known as accommodation.

7. Vitreous Body: A clear, gel-like substance, the vitreous body fills the space in the inner eye between the lens and the retina, called the vitreous cavity. It helps maintain the shape of the eye.

Back of the eye

8. Retina: The thin layer of light-sensing cells lining the back of the eye.

9. Macula: The small orange area near the center of the retina, the macula has a high concentration of photoreceptors that are sensitive to light. It gives you detailed central vision so you can see objects in front of you clearly, which is essential for activities like reading and driving. The macula also enables you to distinguish different colors.

10. Optic Nerve: A bundle of nerve fibers at the back of the eye, the optic nerve sends visual information from the retina to the brain.

How vision works

Starting with light and ending in sight
Vision begins at the cornea, the front part of the eye where incoming light is refracted into the pupil, the dark opening in the eye. The iris, the colored ring surrounding the pupil, opens and closes to control the amount of light entering through the pupil. Once past the pupil, the light passes through the lens, which further refracts the light onto the back of the eye.

Transferring visual information to the brain
A thin layer of tissue called the retina covers the back of the eye. The retina is made of millions of light-sensitive photoreceptors and nerves that detect the image formed by the optics of the eye, much like the pixels of a digital camera. These retinal nerves send information to the optic nerve, which transfers information to the brain. If both eyes function properly, information from the eyes is merged into a single, three-dimensional image by the brain, allowing us to determine the distance of objects we see.

Perfectly focused vision
Normal, perfectly focused vision is called emmetropia, derived from a Greek word meaning “equally measured.” The proportions of the eye’s shape, size and structure have to be exact so that light is brought into sharp focus at the back of the eye. The degree to which the lens of the eye focuses light is called refractive power. Your refractive power is measured in units called diopters, abbreviated “D.” Emmetropia measures 0.0 D.


Living with nearsightedness

If you have myopia, objects in the distance appear blurry, while objects up close are sharply focused. Myopia occurs when incoming light is not accurately focused onto the back of the eye. This may result from the cornea being too curved for the length of the eye.

What is myopia?

Light rays are refracted by the cornea (1) and the lens (2) in such a way that the focal point (3) is in front of the retina (4).

Myopia is the most common sight issue worldwide; in some regions, over half of the population is affected. Also known as nearsightedness, myopia occurs because the eye has too much refractive or optical power in relation to its length. In other words, since the eye is too long, light is brought into focus before reaching the retina. This results in faraway objects appearing blurry because the light is not focused on the retina, where the optic nerve transmits the image to the brain.

Causes of myopia

Myopia usually begins to develop between the ages of 6 and 12. The likelihood of this vision defect is increased if one or both parents are nearsighted. Although everyone is different, typically the degree of myopia in children changes gradually every year—sometimes more rapidly—until later in teenage years, when the severity of the refractive error usually peaks. In rare cases, myopia may continue to progress as a person ages.

While myopia cannot be prevented because the eye condition is hereditary or occurs during growth spurts, research is currently focused on slowing the progression of myopia.

Symptoms of myopia

Common symptoms of myopia include:

  • Headaches from eye strain, for instance after watching TV.
  • Difficulty focusing on distant objects, for instance while driving—particularly at night.
  • Frequent squinting and eye strain when trying to focus on objects in the distance.

Severe myopia can also lead to more degenerative changes in the back part of the eye, such as retinal detachment. That’s why it is important to diagnose and treat myopia. Both ophthalmologists and optometrists can detect the vision defect through a basic eye exam. Myopia is a vision condition, and unless severe, is not a harmful disease. The eye is still healthy but requires visual aids.


Living with farsightedness

With hyperopia, objects up close appear blurry, while objects in the distance are sharply focused. Hyperopia occurs when incoming light emitted from a nearby object is not accurately focused. This may result from the cornea being too flat for the length of the eye.

What is hyperopia?

Light rays are refracted by the cornea (1) and the lens (2) in such a way that the focal point (3) is behind the retina (4).

If you suffer from hyperopia, objects in the distance are usually sharp. However, in severe cases, even distant objects can be blurred. This condition, known as farsightedness, is a result of the eye not having enough optical power.

The cornea and the natural lens normally refract incoming light rays to intersect at a single focal point that lies exactly on the retina—the layer of light-sensitive cells lining the back of the eye. A hyperopic eye refracts light to a focal point behind the retina. This causes vision problems, especially when trying to focus on objects at close proximity. Blurriness occurs because the light is not focused on the retina, where the optic nerve transmits the image to the brain.

Causes of hyperopia

Hyperopia usually develops at birth or at a young age, but only causes visual problems in a small percentage of people until the age of 40, when the percentage starts to increase. At a young age, the flexibility of the natural lens can compensate for hyperopia by changing its shape and focusing the light rays exactly on the retina. This ability is known as accommodation.

Around the age of 40, the natural lens can become less flexible, losing its ability to compensate for the hyperopic refractive error. Also, lower degrees of hyperopia become more noticeable and reading glasses may be needed to focus on close objects.

The condition is often inherited, but in rare cases may occur as a result of various diseases.

Symptoms of hyperopia

If your degree of hyperopia is low, you might not notice your compromised vision. However, living without corrected hyperopia could negatively affect your quality of life. Common symptoms of hyperopia include:

  • Headaches from eye strain after reading, writing or working on a computer for a long period of time.
  • Difficulty concentrating and focusing on objects up close.
  • Eye fatigue or burning eyes.
  • Irritability or anxiousness while trying to concentrate.

It is important to have your eyes examined regularly by an expert. Hyperopia can be diagnosed with a basic eye exam by an ophthalmologist or an optometrist. It is a vision condition, not a harmful disease; the eye is still healthy but requires visual aids.


Objects near and far appear blurred or distorted

SMILE is not approved for astigmatism in the United States.

What is astigmatism?

Astigmatism results from a cornea (1) or natural lens (2) which is not perfectly round. Light rays are focused unevenly onto the back of the eye (3).

If you have astigmatism, the curvature of the transparent front part of your eye, called the cornea, may not be perfectly round. Instead, the shape of the cornea may be more oblong, similar to a football. In contrast, a normal cornea is evenly round, like a basketball.

Astigmatism can be caused by varying degrees of flatness and steepness in the shape of the cornea, depending on the type of astigmatism diagnosed. Due to this irregular shape, incoming light rays are not sharply focused on the retina.

  • Regular astigmatism occurs when the cornea is uniformly curved more in one direction than the other.
  • Irregular astigmatism happens when the curvature of the cornea is uneven across the surface of the eye.

There are three primary types of astigmatism:

  • Myopic astigmatism
  • Hyperopic astigmatism
  • Mixed astigmatism

In all types of astigmatism, light fails to form a single clear image on the retina, the layer of light-sensitive cells lining the back of the eye. In contrast, normal vision has one sharp focal point. Without a focused image on the retina, the information sent to the brain through the optic nerve can result in images that are blurred or distorted—or both—for near and far objects.

Causes of astigmatism

Astigmatism is not preventable. Like myopia and hyperopia, astigmatism is a refractive error, meaning the eye cannot focus light properly onto the retina. It is usually the result of the cornea not being perfectly round.

Astigmatism may also occur if the eye’s natural lens is not round, a condition called lenticular astigmatism. It can also be a symptom of a disease known as keratoconus, which is recognizable because of a very irregularly shaped cornea.

Astigmatism can be present at birth, during early childhood, or could develop after a trauma injury or disease. If one or both parents have the condition, that also increases the likelihood of astigmatism.

Symptoms of astigmatism

Astigmatism can affect sight in different ways, depending on how the cornea is shaped: Vision may be distorted in a horizontal, vertical and/or diagonal direction. This condition can occur by itself or can be accompanied by or in combination with myopia or hyperopia. Typical symptoms of uncorrected astigmatism can be eye strain and headaches that mostly occur after reading or other prolonged visual tasks, such as working on a computer. A very common symptom is frequent squinting.

Regular astigmatism is a vision condition, not a harmful disease; the eye is still healthy. An ophthalmologist or optometrist can detect astigmatism in a basic eye exam. If you are diagnosed with astigmatism, it is likely you will need glasses, contact lenses or corrective laser eye surgery.

Options to correct astigmatism

If you are diagnosed with astigmatism, there are several ways your vision may be corrected. You should consult with an ophthalmologist or optometrist to determine which options are available to you.

Glasses are often a convenient way to improve vision. Prescription glasses can fix refractive errors by altering the angle that light rays enter the eye. There is a wide variety of eyeglass frames and lenses to choose from, suiting different styles and budgets.

Contact lenses correct vision the same way as glasses: fixing refractive errors by altering the angle that light rays enter the eye. Contact lenses differ in materials and how long you can wear them. For astigmatism, specially shaped toric contact lenses are available. It is important to note that contact lenses may present potential complications. Be sure to consult with your ophthalmologist or optometrist about any special considerations.

Laser eye surgery is another option to treat astigmatism. If your degree of astigmatism is moderate to high, LASIK or PRK may be recommended by your doctor. These surgeries have high rates of success. Like all surgeries, these procedures are not without potential risks and possible side effects. Your ophthalmologist or optometrist can give you more information.

Intraocular lens exchange is another option your doctor may recommend for correcting severe astigmatism. This is a surgical procedure in which an artificial lens is implanted in place of the eye’s natural lens. Intraocular lens exchange is a common procedure for patients with cataracts; by comparison, refractive or clear lens exchange is for patients without cataracts. Your doctor can advise whether lens exchange is an option for you as well as which IOL is right for your type of refractive error. Like all surgeries, it is not without potential risks. Be sure to get more information from your ophthalmologist or optometrist.


Age-related farsightedness

Presbyopia is the gradual decline in vision that occurs with age and usually begins around or after the age of 40. The process of aging causes the natural lens to lose its flexibility and thus the ability to focus objects at different distances. As a result, close objects appear blurry.

What is presbyopia?

Presbyopia is an age-related condition that causes nearby objects to appear blurry. At a young age, the natural lens is flexible and can change its shape to focus on both close and distant objects. This process is called accommodation. As people age, accommodation no longer functions to its full degree. The natural lens gradually becomes harder and less flexible, losing its ability to change shape and focus light directly onto the retina. This causes the focal point to lie behind the retina, resulting in a blurry image.

When presbyopia starts

The onset of this condition usually occurs around age 40 but varies from person to person. The decline in your vision might seem sudden, but the loss of flexibility in the natural lens—the cause of presbyopia—progresses over a number of years.

When presbyopia develops and to what degree depends on your individual anatomical eye conditions and any previously existing prescriptions for vision correction or treatment of presbyopia.

Symptoms of presbyopia

Presbyopia can be diagnosed with a basic eye exam by an ophthalmologist or optometrist. You can also recognize presbyopia from some common signs and symptoms, such as:

  • Not being able to read small text or figures.
  • Needing to read menus, smartphones and other written materials and devices with visual displays by holding them at arm’s length.

Other symptoms may include headaches, eye strain and eye fatigue. It is important to visit an ophthalmologist or optometrist for regular eye exams.