20/20 Vision: What is it?Author: Dr. William Tullo
August 1, 2019
20/20 Vision: What does it mean and how to get it?
20/20 vision is a common term that most people have heard of during an eye examination. We all want to achieve 20/20, but what does it really mean?
What is visual acuity and how is it measured?
Visual acuity refers to the level of clarity or sharpness in one’s vision. Typically, visual acuity is determined by the number of lines and/or characters read on the eye chart during an eye examination. Given the results of this type of visual acuity testing is derived from the total characters identified, such measurement methodology is also known as identification acuity.
Dr. Herman Snellen, a Dutch ophthalmologist, first introduced the system of measuring visual acuity via a letter chart in which the letter format and sizing are standardized. Snellen eye charts display same sized characters in each row; these characters are called optotypes and they get smaller towards the bottom of the chart. Visual acuity measurements are recorded as fractions – thus, these test results can also be referred to as Snellen refractions.
Decoding Snellen fraction
The top number of the Snellen fraction represent the testing distance over which one’s visual acuity is measured. This viewing distance is standardized to 20 feet in the US. Although the physical distance between you and the vision chart may not appear to be 20 feet in smaller examination rooms, eye doctors often install mirror system to calibrate an optical distance that simulates 20 feet. This is why the numerator of a Snellen fraction is generally the number 20, ie, 20/20. However, in other countries that uses meters as unit of measurement, an acuity test result of 6/6 is the equivalence of 20/20.
The bottom number of the Snellen fraction represent the lowest line that you were able to identify on the eye chart. Since the character sizes get smaller towards the bottom of an eye chart, visual acuity is better as denominator value in the Snellen fraction gets smaller, ie, 20/20 vision is better acuity than 20/30.
What does 20/20 mean?
On most eye charts, the largest letter(s) on the top row is usually 20/400; whereas 20/20 line can be found near the bottom of the chart. 20/20 vision denotes a level of visual sharpness considered to be the normal baseline in a general population. Another way of interpreting 20/20 is that, on an eye chart positioned 20 feet away, you can see the same small line as those individuals considered to have good normal vision.
Similarly, visual acuity results of 20/40 means that you have to stand at a viewing distance of 20 feet to see the same object that a 20/20 person can see from all the way back at 40 feet – mathematically, that means visual clarity of a 20/40 person is only half as good as someone who sees 20/20.
Why is it important to have good vision?
By convention, 20/20 line is the clinical marker thought to be necessary to meet the visual demands of regular daily activities, which why the ability to see 20/20 or better is important. For example, good vision allows a person to safely navigate one’s environment and perform activities needed to achieve desired quality of life.
The exact level of visual acuity can also influence other facet of lives such as career choices. In order to be a pilot, military personnel or police officer, you need to have at least 20/20 vision. And you will need at least 20/40 vision in almost every state to obtain or renew your driver license.
Is 20/20 the best vision one can get?
While there is no denying that visual clarity can be excellent for those with 20/20 vision, it is possible some people to see better than 20/20.
As previously mentioned, 20/20 line can be found near the bottom of an eye chart. However, that is not the smallest line. Most eye charts have 20/15 and/or 20/10 line(s) as the smallest characters to be identified. A person could have 20/15 or 20/10 vision if he or she can see the smallest letters on a Snellen chart.
Is 20/20 the only thing that matters to my vision?
While 20/20 acuity is not the sole factor that makes our sense of vision the amazing gift that it is, it’s certainly an essential ingredient. Examples of other important visual components may include eye focusing, eye coordination, visual perception, color vision, and binocular vision (how well both eyes work together). Your eye doctors can examine all aspects of your vision during a routine eye health examination and help you keep your eye healthy.
How can my eyesight be corrected to see 20/20 or better?
For those who require corrective prescriptions to improve their visual acuity to 20/20 or better, a refraction procedure, performed during an eye examination by your eye care professionals, will determine the optical powers required to deliver your best vision. The designated lens powers are designed to properly focus the light signals from the physical world to the retinal photoreceptors (cells responsible for sensing light in the back of the eyes), which then send the best quality images to the visual processing center in the brain in order to produce vision.
Once the appropriate optical powers are prescribed, various common treatment methods are available. And it is important to seek the advises of your eye doctors on selecting the most suitable options, based on the results of your eye examinations and your unique visual needs.
- Glasses: The lenses are mounted in a spectacle frame. This is one of the most familiar form of visual correction, which is both practical and convenient for many people,
- Contact lenses: Instead of lenses being mounted into a spectacle frame, prescribed optical powers are incorporated into smaller and biocompatible lens materials that can be worn directly on the eyes. Contact lenses often offer more flexibility to its wearers during daily activities.
- Refractive surgery: LASIK is one of the most common type of refractive surgery performed to further provide freedom during daily activities. And like contact lenses, patients often report improved vision/visual quality after refractive surgical treatments, when compared to wearing glasses.
Other factors that influence my ability to see 20/20 or better
Anatomical structures, such as spatial arrangement of retinal photoreceptors, can determine one’s ability to see 20/20 or better. However, while the photoreceptor arrangement is pre-determined, there are also other physiological factors that can modified to optimize one’s vision. For example, poor tear functions (which can include symptoms of eye redness, dry eyes or even watery eyes) and lack of ocular tissue transparency (ie, development of cataract) can decrease your vision. This highlight the significant importance to have your regular eye exams and follow the recommendations of your eye doctors in order to achieve and maintain your best vision!
Scheduling an annual eye exam allows you to keep up to date with your eye health and make sure your eye prescription is adjusted correctly. Additionally, there are many corrective laser eye procedures that can also get you to enjoy your 20/20 vision!