What Eye Prescription is Legally Blind?
The term "legally blind" actually comes from the government. The U.S. Social Security Administration (SSA) uses this term to determine who is eligible for certain disability benefits, tax exemptions, and low-vision training.
In the United States, a person has legal blindness if:
- Their vision with glasses or contacts is 20/200 or worse
- Their visual field is 20 degrees or less in the better-seeing eye11
This means that a legally blind person has to stand 20 feet away from an object to see it clearly. People with normal vision can stand 200 feet away from that same object and see it clearly.
A legally blind person's vision is at least 10 times worse than someone with normal vision.
Visual Acuity and Eye Prescription
Some legally blind people have visual acuity better than 20/200. However, their peripheral vision is poor.
People with normal vision have a lateral field view of almost 180 degrees. This means they can simultaneously see objects located directly to their left or right.
If your visual field is only 20 degrees, your peripheral vision is reduced drastically. This is often called tunnel vision.
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How Bad Is My Eye Prescription?
No eye prescription should be considered ‘bad.’ Different prescriptions refer to different levels of correction needed to restore normal vision.
The American Academy of Ophthalmology uses the following benchmarks to classify the severity of nearsightedness or farsightedness:
- Mild +/-0.25 to +/-2.00
- Moderate +/-2.25 to +/- 5.00
- Severe +/- 5.00
What Does My Eye Prescription Mean?
An eye prescription is a written order for corrective lenses. An optometrist, ophthalmologist, or optician writes it.
An eye prescription includes:
- Patient’s name
- Date the prescription was issued
- Expiration date
- Specifications for vision correction
- Prescriber’s name, contact number, and signature
Decoding Your Eye Prescription
An eye prescription may include any of the following abbreviations:
- OD: oculus dexter (right eye)
- OS: oculus sinister (left eye)
- OU: oculus uterque (both eyes)
- SPH: the strength of magnification in the lenses
- CYL: cylindrical correction, or the amount of lens power needed to correct astigmatism
- AXIS: indicates the angle of astigmatism correction
- ADD: addition, used for bifocal and multifocal lenses
Focusing Power and Diopters
The numbers listed on the prescription refer to the prescribed focusing power.
Focusing power refers to the eye’s ability to focus on something at a given distance. Its unit of measurement is called a diopter.
A positive number (indicated by a ‘+’ sign) refers to farsightedness or hyperopia. A negative number (indicated by a ‘-’ sign) refers to nearsightedness or myopia.
On an eye prescription, 0.00 represents vision that doesn’t need correction. The numbers indicate the level of severity of your nearsightedness or farsightedness. The further away from zero (+ or -), the worse the eyesight.
Common Causes of Legal Blindness
Many eye diseases can lead to legal blindness. The most common causes of blindness are:10
Cataracts
Cataracts cause clouding of the eye’s lenses. This happens when the proteins in the lens clump together, making it difficult to see clearly. It's the leading cause of blindness worldwide.
Glaucoma
Glaucoma is a disease that damages the optic nerve. Increased pressure inside the eye is the main cause of glaucoma. This condition has no cure, but early intervention can help slow its progression.
Age-Related Macular Degeneration (AMD)
AMD is a disease affecting the retina's macular region. The macular region is responsible for sharp central vision. It's a leading cause of blindness in adults over age 65. When you have AMD, you may gradually lose your central vision.
Diabetic Retinopathy
Diabetic retinopathy is a condition that can affect people with diabetes. Diabetics are at risk of developing blood vessel damage that can lead to poor vision and even blindness.
Is It Bad if My Prescription Changes?
You shouldn't worry about your eye prescription changing over time. Gradual changes to visual acuity are normal as the flexibility of the natural lens changes over time.
These changes can be caused by aging or other eye conditions, such as:
Astigmatism
Astigmatism is a refractive error, like farsightedness or nearsightedness. It causes blurry or distorted vision.
Astigmatism is indicated on a prescription as part of the cylindrical (CYL) correction. If there is no number under CYL, it means there is no astigmatism, or it is so slight that it doesn’t need correction.
Amblyopia
Amblyopia is also called a lazy eye. One or both eyes don’t develop properly during childhood, which affects the eyes’ ability to focus.
Presbyopia
Presbyopia is also called age-related farsightedness. The eyes gradually lose the ability to see objects up close. It typically develops in people over 40 years old.
Sudden vision changes may signify serious eye disease, which should be looked at immediately.
What is 20/20 Vision?
"Normal" vision is 20/20. Someone with 20/20 vision can stand 200 feet from an eye chart and see it as clearly as a legally blind person sees it from 20 feet.
In most states, drivers must have 20/40 vision or better for an unrestricted driver’s license. If you wear eyeglasses or contacts, your corrected vision must be at least 20/40.
Visual Impairment Categories
The World Health Organization uses the following benchmarks to categorize visual impairment:
- Normal: 20/10-20/25
- Near Normal visual impairment: 20/30-20/60
- Moderate visual impairment: 20/70-20/160
- Severe visual impairment: 20/200-20/400, or 11-20 degrees on the visual field
- Profound visual impairment: 20/500-20/1000 visual acuity, or 6- 10 degrees on visual field
- Near-total visual impairment: Counting fingers, Hand motion, Light perception, or 5 degrees or less on visual field
- Total visual impairment: No light perception
Treatment for Bad Eyesight
Treatments for bad eyesight include:
- Eyeglasses. Consists of lenses that are tailored to address specific refractive errors
- Contact lenses. Thin, prescription lenses placed directly on the eye's surface and can serve as an alternative to eyeglasses
- LASIK Surgery. A refractive surgery that aims to reshape the cornea to correct vision problems
- PRK Surgery Another refractive surgery that reshapes the cornea, but doesn't involve creating a corneal flap
- Orthokeratology. A non-surgical treatment that uses specially designed, gas-permeable contact lenses worn overnight
Medical professionals recommend an eye exam every two years for adults ages 19 to 40 with vision problems. Adults older than 40 should get their eyes checked once a year.
Other refractive surgeries are also available for those unsuitable for LASIK and PRK. Consult your ophthalmologist to know more about other treatment options.
Summary
Eye prescriptions change slowly over time. While there's no "bad" eye prescription, you can be "legally blind" when your prescription is 20/200 or worse.
Different diseases can cause bad eyesight, and some treatments can help improve your vision. If you feel that your eyesight is starting to worsen, schedule an eye exam with an ophthalmologist immediately.
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