What Does It Mean to Have 20/400, 20/200, or 20/100 Vision?
By 2050, an estimated 2 million people in the United States are projected to be declared legally blind. One part of the definition of blindness is a visual acuity of 20/200 or less in the good eye with the best possible correction.
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It is often mistakenly believed that someone who is blind cannot see anything at all; however, this is not the case. Vision impairment, or a vision acuity of 20/100 or worse, can affect your functional vision and cause difficulties with seeing things at a distance. It will often require that you use alternative methods to perform daily tasks.
The measurements of 20/100, 20/200, and 20/400 signify how well you can see things at distances of 20 feet with the lowest number being the best. For example, 20/20 is considered perfect vision.
The Definition of Legally Blind
In the United States, the definition of legally blind is twofold. The first part involves visual acuity. This is based on a standard eye test using an eye chart like the Snellen Eye Chart.
You will stand 20 feet away from the chart and read down the lines. Your visual acuity is based on when you can no longer read the lines.
On the Snellen Eye Chart, the first line typically contains only one letter and is the determination for 20/200 vision. If you cannot read this line, it means that your visual acuity is 20/200 or worse, which is part one of the definition of legal blindness. The 20/200 prescription equivalent is -2.5.
Visual acuity is based on how well you can see out of your best eye and with standard corrective lenses, such as prescription eyeglasses or contact lenses. If you are only able to see at 20/200 or lower, even with corrective lenses, you can be diagnosed as legally blind.
The second part of the designation of blindness is based on your field of vision, which involves how well you can see to the sides without moving your eyes. Visual acuity eye charts test your central vision.
There are alternate methods for testing visual field. If your field of vision is 20 degrees or less, you are considered legally blind. This constitutes tunnel vision, as you will have trouble seeing things that are not directly in front of you.
Understanding the Differences Between 20/100, 20/200 & 20/400 Vision
Normal visual acuity is considered to be 20/20 vision, which is line 8 on the Snellen Eye Chart. This means that from 20 feet away, you can read that line clearly without correction.
The first line on the Snellen Eye Chart is for 20/200 vision, while line 2 is for 20/100 visual acuity. This means that you will need to be 20 feet away to see something as clearly as someone with normal vision can see with the same clarity at 100 feet away.
If you can read the second line from 20 feet away but not the first, your visual acuity is 20/100. If you cannot read the first line, your visual acuity is at least 20/200 and potentially worse.
It will take a specialized low vision eye chart to determine your exact visual acuity if it’s worse than 20/200. A specialized eye chart can also give you more precise results between 20/100 and 20/200.
Visual acuity between 20/200 and 20/400 is classified as severe vision loss, and anything below 20/400 is considered profound vision loss. Low vision testing will typically involve finger counting or specialized exams to determine visual acuity below 20/400, as you will be unable to read a standard eye chart.
A visual acuity of 20/400 is very low functional vision. The lower your functional vision, the harder it will be to do daily activities and perform everyday tasks without low vision aids, specialized tools, or services.
To better understand your visual acuity and how to manage your visual needs, talk to your eye doctor and schedule a comprehensive eye exam.
References
- The Burden of Vision Loss. (June 2020). Centers for Disease Control and Prevention (CDC).
- Low Vision and Legal Blindness Terms and Descriptions. American Foundation for the Blind (AFB).
- All About the Eye Chart. (November 2016). American Academy of Ophthalmology (AAO).
- Visual Acuity. American Optometric Association (AOA).
- The Low Vision Examination. (2020). Vision Aware.