Emmetropia vs Ametropia (Definition & Overview)

10 sources cited
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Emmetropia vs. Ametropia

Emmetropia is a medical term for "normal vision," or what doctors call 20/20 vision. On the other hand, ametropia refers to the presence of one or more refractive errors (visual defects).

What is Emmetropia?

If your eye doctor says you have emmetropic eyes, they mean you have "20/20 vision." 20/20 vision means you can see an object of a specific size clearly at 20 feet. The images are perfectly focused. Therefore, emmetropic eyes do not require any vision correction.

How Does Emmetropia Occur?

Emmetropia occurs when the length of your eye and its optical ability are in perfect balance, resulting in 20/20 vision. It’s not a medical condition. 

Most children are born hyperopic (farsighted).1 For those born hyperopic, the condition typically improves to reach emmetropia by the time they attain toddler age (1 to 3 years old). The process of developing an emmetropic eye is referred to as emmetropization

What’s the Science Behind Emmetropia?

The science of emmetropization is not well understood, but experts have pointed to the influence of the environment’s visual input, genetics, and brain activity. 

Understanding how infants develop emmetropic vision can help scientists develop new ways of correcting or preventing refractive errors.

According to research, only 35% of adults have 20/20 vision without corrective measures such as sunglasses, contact lenses, or eye surgery.2 However, about 75% of adults achieve 20/20 vision after correction. 

What Happens in Emmetropia?

The following happens in emmetropic eyes:

  1. Light enters the eyes via the pupil (the black hole at the center) and focuses on the retina. The retina is the light-sensitive part at the back of the eye. It contains special cells known as photoreceptors (rods and cones).3 
  2. These photoreceptors receive and transform light into electrical signals, which are then transmitted to the brain via the optic nerve. 
  3. In the brain, the signals are interpreted, enabling you to see clearly and understand what you see.

If a person has emmetropic eyes, light rays entering the eye from a distance are directly focused on the retina, resulting in normal vision (emmetropia), otherwise, they will have ametropia. 

What is Ametropia?

Ametropia means you have one or more refractive errors. Common examples of refractive errors include:

Myopia (nearsightedness)

Someone with myopic eyes has blurry distance vision and clear near vision.4 With myopia, abnormally shaped eyeballs cause light to be refracted incorrectly when entering the eyes. In this case, the light is focused in front of the retina instead of directly on it. 

Myopia is commonly genetic, meaning it's passed down the family line. It usually affects children between ages 6 and 14 and worsens as they age.5 

Other symptoms include:

  • Eye strain leading to headaches
  • Poor night vision 
  • Needing to squint to see clearly (e.g., when watching television)
  • Excessive blinking
  • Eye rubbing

Hyperopia (farsightedness)

Hyperopic people can see distant objects clearly, whereas nearby objects appear blurry.6 In addition, the ability to focus on an object is influenced by the severity of the condition. 

For example, severely hyperopic people cannot see objects clearly at a great distance or at near, whereas mildly hyperopic ones can see better at a distance

Hyperopia results from an abnormal corneal curvature or shorter eyeball (from front to back). This condition is genetic and usually presents at birth. Other symptoms include:

  • Needing to squint to see clearly
  • Eye strain
  • Burning/itching/aching eyes
  • Headaches after close tasks such as reading or computer work

Presbyopia

Presbyopia is age-related farsightedness. It's a normal part of the aging process and also affects people who have been emmetropic their entire life, usually after 45 years.7 As you age, the natural eye lens loses its flexibility and stops focusing light correctly on the retina.

The condition worsens over time and usually stabilizes after age 65. Common symptoms include:

  • Trouble seeing up-close objects
  • Eye strain
  • Headaches
  • Needing to hold reading materials further away to see clearly

Astigmatism (irregularly shaped cornea)

People's eyes develop astigmatism when the cornea (clear front part of the eye) or the natural eye lens has an abnormal shape, making light bend incorrectly when entering the eye. 

Some people are born astigmatic, while others may develop the condition due to genetics, eye trauma, surgery, or other causes.8

Someone with mild astigmatism may not notice any symptoms. This is why regular eye check-ups are important, especially for children who are less likely to realize visual abnormalities. Common symptoms include:

  • Blurry vision
  • The need to squint to see clearly
  • Eye strain accompanied by headaches
  • Poor night vision

How Does Ametropia Occur?

Ametropia occurs when your eyes are unable to refract light correctly. This may cause inability to see distant or nearby objects clearly. If you have an abnormal eye length or the cornea (the clear front part) is abnormally shaped, emmetropization will not be possible.

The nature of the refractive defect determines your type of ametropia. Eyes that are longer than average will cause light to be focused in front of the retina, causing nearsightedness (myopia).

On the other hand, if the eyeball is shorter than average, light rays will be focused behind the retina, causing farsightedness (hyperopia). All these can be categorized as ametropia.

What is High Ametropia?

High myopia is a medical term for severe myopia (nearsightedness) measuring more than 6.00 diopters (D). 

High ametropia is caused by an excessively elongated eyeball, an extremely curved cornea, or a combination of the two. 

The condition generally begins during childhood and worsens over time with an increased risk of glaucoma, cataracts, or retinal detachment.9

How to Protect Your Eyes

Whether you have emmetropia or ametropia, protecting your eyes is important. You can protect your eyes and maintain healthy vision by:

  • Attending regular eye check-ups, even if you have good eyesight (mild symptoms may go unnoticed)
  • Managing chronic conditions such as diabetes and hypertension, which can affect vision
  • Wearing hats and UV sunglasses to prevent direct sunlight
  • Using protective gear when playing a contact sport or engaging in risky activities such as mowing the lawn
  • Avoiding smoking
  • Using corrective lenses to optimize your vision
  • Avoiding digital eye strain by observing the 20-20-20 rule
  • Avoiding poor lighting, which can strain your eyes

Visit your ophthalmologist, optometrist, or optician urgently if you experience blurry vision, double vision, flashes of light, or halos around lights. These may indicate a serious medical or eye condition.

Summary

  • Emmetropia refers to "normal vision," or what doctors call 20/20 vision
  • On the other hand, ametropia is the presence of a refractive error
  • Emmetropia occurs when the length of your eye and its optical power are in perfect balance, resulting in clear, precise, and well-focused vision
  • If your eyes do not focus light well on the retina, ametropia occurs
  • You can protect your eyes from ametropia or slow its progression through regular eye check-ups or observing healthy lifestyle habits
10 Cited Research Articles
  1. Forrester. “Emmetropia,” Science Direct, 2021.
  2. American Academy of Ophthalmology. “What Does 20/20 Vision Mean?,” www.aao.org, 2022.
  3. National Eye Institute. “How the Eyes Work,” www.nei.nih.gov, 2022.
  4. Mayo Foundation for Medical Education and Research (MFMER). “Nearsightedness,” www.mayoclinic.org, 2020.
  5. National Eye Institute. “Nearsightedness (Myopia),” www.nei.nih.gov, 2020.
  6. American Academy of Ophthalmology. “Farsightedness: What Is Hyperopia?,” www.aao.org, 2022.
  7. U.S. National Library of Medicine. “Presbyopia,” medlineplus.gov.
  8. National Eye Institute. “What is astigmatism?,” www.nei.nih.gov, 2019.
  9. The Eye Disease Case-Control Study Group. “Risk Factors for Idiopathic Rhegmatogenous Retinal Detachment,” American Journal of Epidemiology, Volume 137, Issue 7, 1993.
  10. American optometric Association. “20-20-20 To Prevent Digital eye Strain,” www.aoa.org.
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