What is emmetropia?

What is emmetropia?

Definition of emmetropia

Emmetropia sounds like an important medical concept or health issue (try saying it while looking up dramatically from your imaginary hospital chart). But it’s really just the state of normal vision.

When someone says they have 20/20 eyesight, that’s emmetropia. So, if you happen to have 20/20 eyesight, feel free to start spreading the rumor that you've got emmetropia, and watch everyone wonder what mysterious condition might be afflicting you.

For those without emmetropia (perhaps you, since you’re reading a vision care blog), common visual conditions include refractive errors such as myopia (nearsightedness) and hyperopia (farsightedness). Fortunately, corrective lenses and contacts can help get vision closer to the ideal, making images appear clearer and more focused.

Understanding visual acuity

Visual acuity is a measure of how clear and crisp objects appear at standard distances. The biggest factor affecting visual acuity is the shape and position of your retina, the part of your eye where image focus occurs.

In normal eyes, the visual signals coming in hit the retina just right, so you see everything clearly. 

But when that visual signal comes in slightly forward or behind the retina, things begin to look blurry. This can occur if the eyeball is slightly shorter or longer than normal. 

Decoding 20/20 vision

The term 20/20 vision is typically used to describe standard normal visual acuity. In the simplest terms, it means that you can see things at 20 feet that anyone else with normal vision could see at 20 feet.

People with refractive errors might need to be closer or further away than 20 feet to make out the same object, which is where corrective lenses come in. Someone with 20/20 vision doesn't need glasses or contacts to correct blurry vision, because nothing looks blurry.

So how can you tell if you’ve got 20/20 vision? The Snellen visual acuity test. You're probably familiar with this particular eye test, which features lines of gradually smaller letters that you attempt to read aloud from across the room. During a Snellen eye test, you try reading the chart with one eye, and then the other, before attempting the feat with both eyes open. 

Fun fact: The numerator, or first number of the results, indicates the distance in feet between you and the test chart, so it's always set at 20. The denominator, or second number after the slash, is the distance where someone with normal vision could read the same line on the chart. 

For example, someone with worse visual acuity than normal might have 20/30 or 20/40 vision. They only see at 20 feet what someone with normal vision can see from 30 or 40 feet away. Test results of 20/200 or more are used to diagnose someone as legally blind.

Exploring 20/15 vision

Some people have better vision than average, which can give visual acuity test results of 20/15. If you've got 20/15 vision, you can see things at 20 feet that someone with normal vision sees at 15 feet. Details may be sharper, and you may experience a clarity of vision that keeps the world looking well-defined no matter where you look. You’re basically a superhero!

But while you may be great at spotting parking spaces, it doesn't mean you won't have other eye issues. Peripheral vision, color blindness, and binocular vision are some aspects of vision unaffected by emmetropia. And yes, even people with 20/15 vision sometimes need glasses or contact lenses.

Factors affecting visual acuity

Refractive errors and visual acuity

Refractive errors affect visual acuity. Most present as blurry vision, but the distance where you see blurriness determines the exact diagnosis. Common types of refractive errors include:

• Hyperopia. Also known as farsightedness, hyperopia occurs when distant things are seen clearly, but nearby objects become blurry. 
• Myopia. Also known as nearsightedness, myopia is the opposite of hyperopia. You can see objects up close, but have trouble focusing on faraway things.
• Astigmatism. An astigmatism develops if the cornea has an irregular curvature. This can cause blurring of both near and far objects. 

Refractive surgery, also known as laser eye surgery, is often done to correct refractive errors. Glasses and contact lenses also serve as corrective measures that improve visual acuity. For eye problems unrelated to visual acuity, you may need vision therapy.

Other factors affecting visual acuity

Age can affect visual acuity, and people often notice changes in vision as they get older. Presbyopia, an inability to focus on objects close to you, is a common problem that develops in older adults. If you've noticed increasing difficulty reading the instructions on food packages or medicine bottles, it's probably presbyopia—no matter how much you want to blame the company for changing their font size.

Certain other vision conditions, including cataracts and macular degeneration, can also affect visual acuity. Amblyopia, which develops when the signals between your brain and eyes aren't properly coordinated, is another eye issue that can occur even when the retina is working properly.

Regular eye exams help identify these and other conditions early, so you can get an accurate glasses or contacts prescription and maintain healthy vision.

Understanding emmetropia and maintaining visual acuity 

Emmetropia isn't quite the same thing as “perfect” vision, but it is considered healthy, standard vision. A visual acuity of 20/15 or 20/10 gives you impressive eyesight, but it doesn't mean other problems might not occur. 

Regularly scheduled vision exams make it easy to monitor any changes in your visual acuity, or detect other problems that could affect your eyes.

Noticing a lack of emmetropia in your life? Corrective measures, like prescription glasses and contacts, can help keep everything crystal clear, while laser vision surgery is a more permanent fix for refractive eye errors.

If you already have a prescription for contacts, find your brand at 1-800 Contacts and get them shipped free, right to your door.

 

FAQs

Is it possible to achieve better than 20/20 vision?

Some people naturally have better than 20/20 vision, but it's not really possible to alter your visual acuity in a way that increases it to that level. 

Can visual acuity change over time?

Visual acuity can worsen over time as the eyes change with age, but this usually doesn't occur until after about age 40. Many older people find it harder to read small print, and close-up objects seem blurrier. Your eyes might also have more difficulty adjusting to changes in light conditions as you age.

What are the common causes of reduced visual acuity?

Age is one of the biggest causes of reduced visual acuity. Health conditions such as diabetes, thyroid issues, and diseases affecting the cornea can also impact your vision, along with eye strain.

Does 20/20 vision mean perfect vision in all aspects?

It's important to remember that 20/20 vision isn't the same thing as perfect vision. Other things can affect your eye health, and even someone with 20/20 vision might develop issues such as macular degeneration or cataracts.