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Refractive Errors

What is normal vision? Answer

What does it mean when someone has 20/20 vision? Answer

What is legally blind? Answer

What Does It Mean to Have 20/400, 20/200, or 20/100 Vision? Answer

What is visual impairment? Answer

What is functional blindness? Answer

What are the primary types of vision impairment and blindness? Answer

What are refractive errors? Answer

What are the types of refractive errors? Answer

What causes refractive errors? Answer

What is myopia (nearsightedness)? Answer

What is hyperopia (farsightedness)? Answer

What is astigmatism? Answer

What is presbyopia? Answer

What are the symptoms? Answer

Is myopia the same as nearsightedness? Answer

What causes myopia? Answer

How is myopia treated? Answer

Is myopia preventable? Answer

What is the difference between reading glasses and distance glasses? Answer

How often and how long should plus lenses be used? Answer

What if problems such as tired eyes are experienced when using plus lenses? Answer

Can myopia be prevented or improved by using eye exercises? Answer

For those who are already nearsighted, will the eyes get better? Answer

Why do many people need stronger minus glasses year after year? Answer

Are plus lenses harmful in any way? Answer

What is a Phoropter? Answer

How long have phoropters been around? Answer

Prescription Eyeglasses

What is the Difference between an Ophthalmologist, an Optometrist and an Optician? Optometrist, Ophthalmologist, Optician: What’s the Difference? Answer

Why do Optometrists write Rx’s in negative cylinder while Ophthalmologists use positive cylinder? Answer

What is the latest trend in eyeglasses? Answer

What do all those numbers on your eyeglass prescription mean? Answer

Do contact lens prescriptions differ from those for eyeglasses? Answer

What is normal vision?
Vision or visual acuity is tested by reading a Snellen eye chart at a distance of 20 feet. If you have 20/20 vision that means when you stand 20 feet away from the chart you can see what a “normal” human being can see at 20 feet. If you have 20/40 vision, it means that when you stand 20 feet away from the chart you can only see what a normal human can see when standing 40 feet from the chart.

Likewise, 20/200 means that when a person stands 20 feet from a chart he can only see what a normal person standing 200 feet away can see. The cutoff for legal blindness in the United States is 20/200.

It also is possible to have vision better than normal. A person tested to have 20/10 vision can see at 20 feet what a person with normal vision can see when standing 10 feet from the chart.

What does it mean when someone has 20/20 vision?

By looking at lots of people, eye doctors have decided what a "normal" human being should be able to see when standing 20 feet away from an eye chart. If you have 20/20 vision, it means that when you stand 20 feet away from the chart you can see what the "normal" human being can see. (In metric, the standard is 6 meters and it's called 6/6 vision). In other words, your vision is "normal" -- most people can see what you see at 20 feet. (From here on, please assume that the word "normal" has quotes around it).

If you have 20/40 vision, it means that when you stand 20 feet away from the chart you can see what a normal human can see when standing 40 feet from the chart. That is, if there is a normal person standing 40 feet away from the chart and you are standing only 20 feet away from it, you and the normal person can see the same detail. 20/100 means that when you stand 20 feet from the chart you can see what a normal person standing 100 feet away can see. 20/200 is the cutoff for legal blindness in the United States.

ÂYou can also have vision that is better than the norm. A person with 20/10 vision can see at 20 feet what a normal person can see when standing 10 feet away from the chart.

Hawks, owls and other birds of prey have much more acute vision than humans. A hawk has a much smaller eye than a human being but has lots of sensors (cones) packed into that space. This gives a hawk vision that is eight times more acute than a human's. A hawk might have 20/2 vision!

What is legally blind? What Does It Mean to Have 20/400, 20/200, or 20/100 Vision?
The formal description of legal blindness is: “Central visual acuity 20/200 or less in the better eye with best correction, or widest diameter of visual field subtending an angle of no greater than 20 degrees.”

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.

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.

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.

What is visual impairment?
A person is considered to have low vision or visual impairment if their best corrected vision is 20/40 or worse. This person isn’t considered legally blind, but they can have difficulty in certain visual environments.

What is functional blindness?
A person is functionally blind when they have to use alternative techniques to perform tasks, such as reading using braille.

What are the primary types of vision impairment and blindness?
Central vision loss
Most of your fine detailed vision is in the center of your visual field. This includes reading. Eye conditions that can interfere with your central vision include:
macular degeneration
Stargardt disease
Tunnel vision

Tunnel vision allows you to see in the central part of your vision, but not in the periphery. Tunnel vision interferes with travel vision more than it does with reading vision. Eye conditions that can cause tunnel vision include:
glaucoma
retinitis pigmentosa
Blind spots

Some people have vision with blind spots. These can be caused by traces of blood in the vitreous fluid in the eye that block certain areas. The amount and location of vision impairment can change daily. Conditions that can cause blind spots include:
diabetic retinopathy
inflammation of the optic nerve
Partial vision

Also known as low vision, partial vision varies from person to person, depending on factors such as:
color
glare
movement
fatigue
lighting
size

Some people with partial vision could be considered legally blind. Conditions that can cause partial vision include:
cataracts
retinitis pigmentosa
retinopathy of prematurity
Total blindness

While some people have absolutely no vision, some people who are considered totally blind may have light perception or the ability to see bright colors or some movement. Conditions that can cause total blindness include:
glaucoma
diabetic retinopathy
macular degeneration

What are refractive errors?
Refractive errors are a type of vision problem that makes it hard to see clearly. They happen when the shape of your eye keeps light from focusing correctly on your retina (a light-sensitive layer of tissue in the back of your eye).

Refractive errors are the most common type of vision problem. More than 150 million Americans have a refractive error — but many don’t know that they could be seeing better. That’s why eye exams are so important.

If you have a refractive error, your eye doctor can prescribe eyeglasses or contact lenses to help you see clearly.

What are the types of refractive errors?
1. Astigmatism
2. Hyperopia (Farsightedness)
3. Myopia (Nearsightedness)
4. Presbyopia (Age-Related Farsightedness)

Refractive errors include astigmatism, hyperopia, myopia, and presbyopia, eye conditions that are very common.

There are 4 common types of refractive errors:
Astigmatism can make far-away and nearby objects look blurry or distorted
Farsightedness (hyperopia) makes nearby objects look blurry
Nearsightedness (myopia) makes far-away objects look blurry
Presbyopia makes it hard for middle-aged and older adults to see things up close

What is Good Visual Acuity?
The term 20/20 is used to express normal visual acuity measured at a distance of 20 feet. If you have a vision of 20/20, you can see clearly at the distance of 20 feet.

What Affects Visual Acuity?
Visual acuity is limited by aberrations, diffraction, and photoreceptor density inside the eye. Apart from these limitations, a number of factors also affect visual acuities, such as refractive error, illumination, contrast, and the location of the retina being stimulated.

Impact of decreased visual acuity on daily function:
 Decreased facial recognition
 Difficulty finding one’s way through a crowded environment
 Difficulty reading daily schedule
 Difficulty with reading

What is myopia (nearsightedness)?
If you have myopia you can clearly see close objects, but distant objects are blurry. Myopia is caused by the eyeball being too long. Myopia occurs in different degrees from minimal to extreme. The more myopic you are the blurrier your vision is at a distance and objects will have to be closer to you so you can see them clearly.

What is hyperopia (farsightedness)?
If you have hyperopia, you can see distant objects clearly, but close ones are blurry. Hyperopia occurs when the eyeball is too short for the light rays to focus clearly on the retina.

What is astigmatism?
If you have an astigmatism, the surface of the eye (cornea) is not perfectly round, rather it is more oval and doesnÂ’t allow the eye to focus clearly. The cornea is very important in helping the eye focus light rays on the retina. Astigmatism rarely occurs alone. It is usually accompanies myopia or hyperopia.

What is presbyopia?
If you have presbyopia, you have the loss of the ability to focus up close that occurs as you age. Most people are between 40 and 50 years when they realize for the first time that they canÂ’t read objects close to them. The letters of the phonebook are Â“too smallÂ” or you have to hold the newspaper farther away from your eye to see it clearly. At the same time your ability to focus on objects that are far way remains normal.

HOW IS MYOPIA TREATED?
Nearsightedness may be corrected with glasses, contact lenses or refractive surgery. Depending on your vision problem, you may need to wear your glasses or contact lenses all the time, or only when you need distance vision, like driving, seeing a chalkboard, or watching a movie.

What are the symptoms?
The only symptom of myopia is the blurred vision for distance. The fact that young children tend to hold everything close to their face or sit very close to the television does not necessarily mean they are nearsighted. All children tend to do this because they see so much better up close. Eye fatigue, burning eyes, headache, and limited tolerance for reading are not usually symptoms of myopia.

Is myopia the same as nearsightedness?
Yes.

What causes myopia?
Children are normally born farsighted and must exert focusing effort (accommodate) even for distance. This causes a ciliary muscle spasm followed by an eye elongation that reduces the farsightedness and accommodative effort. This is nature's way of refining the length of the eye so that excellent vision is the end result. The process is supposed to stop when only a little farsightedness remains. It is constant close work which causes this process to go too far and move the eye into myopia. If we did not learn to read, we would not become nearsighted

Is myopia preventable?
We believe that the answer is yes. If children use reading glasses for reading and other close work as soon as the first signs of nearsightedness appear, myopia can be prevented.

What is the difference between reading glasses and distance glasses?
Reading glasses, also known as plus lenses, are convex in shape. They move objects farther away and reduce accommodation (focusing effort). Distance glasses, also known as minus lenses, are concave in shape. They move objects closer and increase accommodation.

What if an optometrist or ophthalmologist agrees with the benefit of plus lenses but does not want to recommend a strong enough lens to completely eliminate focusing when reading?

Weak reading glasses are better than nothing and far better than distance glasses for the child who is beginning to experience myopia, but studies have shown that weak reading glasses are usually not enough to prevent myopia.

How often and how long should plus lenses be used?
Plus lenses are used to protect our eyes from the unnatural demands of modern life. They should be used for all PROLONGED reading, computer use and other close work. When the school years are over, the glasses can usually be put aside, since the adult eye tends not to elongate further. The glasses can be saved for middle age when the focusing ability diminishes and they are needed for reading once again!

What if problems such as tired eyes are experienced when using plus lenses?
Simply stop using them for a while.

If a child looks into the distance while wearing reading glasses, won't everything seem even more blurred?
Yes. That's why they should be taken off when not reading.

Are bifocals a good alternative?
Bifocals are a combination of reading glasses and distance glasses. The upper portion of the glasses contains distance (minus) lenses and the lower portion contains reading (plus) lenses. We do not recommend bifocals because the child is likely to use the distance lenses in the upper half of the glasses for close work, thereby further elongating the eye and causing irreversible myopia. As an example, when using a computer, the child may use the lower portion to look at the keyboard, but will almost certainly use the upper portion to look at the screen.

Can myopia be prevented or improved by using eye exercises?
No. The myopic eye is not in need of "exercise." It is in need of relaxation. When we look into the distance, our eyes are at rest. Focusing close is stressful for the eyes.

For those who are already nearsighted, will the eyes get better?

Studies have shown that by relaxing the ciliary muscle spasm through the vigorous use of plus lenses as much as one diopter of myopia can be eliminated. If the amount of myopia is greater than that, elongation of the eye has probably occurred. This cannot be reversed. Therefore, all of the myopia cannot be eliminated.

Many people don't mind wearing glasses. They think they are "cool." Why should they care about prevention?

Myopia leads to an increased danger of cataracts, glaucoma, macular degeneration and retinal detachment. The more myopia one has, the more that risk increases. It also leads people to undergo risky corneal surgery to reduce the myopia. Many people have lost their sight from such surgery.

Why do many people need stronger minus glasses year after year?
Because each time they are increased in power they cause more eye elongation and the need for stronger glasses.

Are plus lenses harmful in any way?
No. In fact they are sold in drug stores to young and old alike without the need for a prescription.

How far should the eyes be from the book or computer or other close object when using the reading glasses?
When using the reading glasses, the child should keep the close object as far away as possible. It is best to keep the object far enough away so that it appears very slightly blurred, but still legible. As the eye tries to clear this slightly blurred image, the maximum relaxing force is applied to the ciliary muscle.

Can a child just use reading glasses bought at a drug store?
No. A professional should be seen to obtain individualized advice.

Drug store reading glasses are intended for adult-size heads. The distance between the lens centers conforms to the usual distance between the pupils of the adult head. If such glasses are used by young children, who have a smaller interpupillary distance, the "prismatic effect" of the lenses causes increased convergence. This can cause problems such as double vision and should be avoided. The distance between the child's pupils should be measured when the eyes are converging on a book. The centers of the eyeglass lenses should not be any farther apart than that distance.

Spectacles
Fashionable Frames
Quality Lenses
Unbeatable Price

What are Polarised lenses?
Polarised lenses give maximum protection from reflected glare and the sun's powerful UV rays. They improve clarity, so you are able to see contrast in colours and have greater perception of depth. They are especially good for fishermen, because you can see through water, but they are just as valuable for normal sun wear, water-sports, golfing, shooting and many other activities. They are also tougher than standard lenses.

What are Transition lenses?
Transition lenses are light sensitive plastic lenses that react very quickly to UV light so they darken in the sun and become clear indoors. They give you 99.9% UV protection from bright sunlight and have a scratch resistant coating. Having them fitted means that you donÂ’t have to have a separate pair of sunglasses.

What are Polycarbonate lenses?
Polycarbonate lenses are plastic lenses that are ten times more impact resistant than other plastics making them very suitable for contact sports or butter fingers! They offer UV protection and stop 99.9% of rays from the sun, fluorescent lights and computer screens. They come with a scratch resistant coating, are thinner than standard lens and therefore more comfortable to wear.

What is a Phoropter?

Ophthalmologists and optometrists, especially those who handle eye tests, will most often use a phoropter, to enable an accurate spectacles’ prescription for a patient.

A phoropter is used to assist with finding the right prescription for nearsightedness, farsightedness, or astigmatism. Your eye doctor will use the multiple lenses in the phoropter to find the right strength that works with any issues you may be having with your eyesight. With the help of a phoropter, your eye doctor can write up the appropriate prescription for your next pair of glasses.

How long have phoropters been around?
The first versions of the phoropter can be seen in the early 1900’s, when Nathan Shigon invented the “Ski-optometer” and Henry DeZeng developed the “Phoro-optometer.” From there, this equipment continued to evolve until it eventually became the phoropter you are familiar with today.

Eyeglasses
Spectacles
How to Read Your Eyeglasses Prescription

What is the Difference between an Ophthalmologist, an Optometrist and an Optician? Optometrist, Ophthalmologist, Optician: What’s the Difference?
Ophthalmologists are medical doctors who specialize in eye disease.
Optometrists focus on refractive error as well as treatment and maintenance of eye abnormalities such as glaucoma. If the optometrist determines that a patient has an eye disease beyond the scope of what they can treat, or if they determine that surgical treatment to correct a problem may be necessary, they will refer to an ophthalmologist.
An optician is trained to fabricate and fit eyewear.

Why do Optometrists write Rx’s in negative cylinder while Ophthalmologists use positive cylinder?
These are different ways to describe astigmatism correction in glasses. Ophthalmologists work in “plus” cylinder while optometrists like “minus” cylinder. Mathematically, they are the same.

Glasses are made to correct a certain amount of spherical error. However, people also have a certain amount of astigmatism that also needs to be corrected. Astigmatism is when the surface of the cornea is not perfectly round like a basketball, but more round like a football. The extra steepness along one axis needs to be corrected with a cylinder.

The cylinder correction can either be made with a positive convex lens or a negative convex lens, the end result is the same.

Eyeglass Prescription

Prescriptions are written differently depending on whether it was from an Optometrist or an Opthamologist.

An Optometrist O.D. is a health care professional who is licensed to prescribe glasses, contact lenses and provide low vision rehabilitation. Examine and diagnose common eye diseases and to inspect, diagnose and treat visual conditions.

An Ophthalmologist M.D. is a medical doctor who is trained to provide complex and delicate eye surgery. They examine, diagnose and treat eye diseases such as glaucoma, cataracts, and retinal diseases. They may also be involved in eye research.

Optometrist vs. Ophthalmologist

An Optometrists prescription is written in minus (-) cylinder and an Ophthalmologists prescription is written in plus (+) cylinder. The following prescriptions have the same result but look different because of the way they were written.

Optometrists Prescription
Cylinder power has a minus (-) sign

Sphere		Cylinder	Axis
R	-2.25	-0.75	125
L	-2.00	-1.25	085

Ophthalmologists Prescription
Cylinder power has a plus (+) sign

Sphere		Cylinder	Axis
R	-3.00	+0.75	035
L	-3.25	+1.25	175

More examples of how prescriptions can be written

A plus (+) or minus (-) sign should be included in front of all numbered sphere and cylinder values.

Minus Sphere Only Prescription
No cylinder or astigmatism correction
SPH stands for sphere only, no cylinder.
Sphere only prescriptions have no Axis value.

Sphere		Cylinder	Axis
R	-2.25	SPH
L	-2.00	SPH

Plus Sphere Only Prescription
No cylinder or astigmatism correction
DS stands for diopter sphere only, no cylinder.
Sphere only prescriptions have no Axis value.

Sphere		Cylinder	Axis
R	+2.25	DS
L	+2.00	DS

Minus on Minus Prescription
Sphere and Cylinder are both minus (-)

Sphere		Cylinder	Axis
R	-2.00	-0.75	107
L	-2.00	-1.25	085

Plus on Minus Prescription
Sphere plus (+) and Cylinder minus (-)

Sphere		Cylinder	Axis
R	+2.00	-0.75	107
L	+2.00	-1.25	085

Minus on Plus Prescription
Sphere minus (-) and Cylinder plus (+)

Sphere		Cylinder	Axis
R	-2.00	+0.75	107
L	-2.00	+1.25	085

Plano Sphere with Cylinder Prescription
Sphere Plano (0) and Cylinder minus (-)

Sphere		Cylinder	Axis
R	PL	-0.75	107
L	+0.50	-1.25	085

What is the latest trend in eyeglasses?
What do all those numbers on my eyeglass prescription mean?
An eyeglass prescription is written in a standardized format so it can be interpreted worldwide. Let's look at one and see what all the numbers mean:

Let's say the eyeglass prescription for your right eye is: -2.00 -1.00 x 180.

The first number (-2.00) indicates the spherical lens power to correct nearsightedness or farsightedness. If the number is preceded by a minus sign (as it is here), this indicates a lens power to correct nearsightedness. If the number were preceded by a plus sign (+), then the lens power would be one to correct farsightedness. In all cases, the unit of power for numbers in an eyeglass prescription is called a diopter (D). So this eyeglass prescription is calling for the correction of 2.00 D of nearsightedness.

In all cases, the unit of power for numbers in an eyeglass prescription is called a dioptre (D). So this eyeglass prescription is calling for the correction of 2.00 D of short sightedness.

The second number (-1.00) is the supplemental lens power (called "cylinder" power) being prescribed for the correction of astigmatism. Cylinder power can be preceded by a minus sign or a plus sign, depending on the prescribing style your eye doctor chooses. If you have no need for astigmatism correction, your doctor might simply draw a horizontal line through this box on your prescription or write "SPH" or "DS" to indicate that only spherical power to correct short sightedness or long sightedness is needed.

The third number (x 180) indicates the location of what's called the "axis" of the astigmatism correction. Unlike spherical lens powers that correct nearsightedness or farsightedness, cylinder powers to correct astigmatism have a different amount of power in different meridians of the lens. (Think of meridians like the spokes of a wheel.)

The axis of astigmatism is the meridian of the cylinder that has zero power, and it always will be a number between 1 and 180 on your prescription, preceded by an "x." If you have no astigmatism, this part of your eyeglass prescription will be left blank.

If you have presbyopia and need bifocals or other multifocal lenses, your eyeglass prescription will contain a number in a box marked "ADD". This is the additional magnifying power placed in the lower half of your lenses to improve your reading vision. It might be preceded by a "+" sign, and it typically will be a number between 0.75 and 3.00.

Finally, you may see the notations "OD" and "OS" on your eyeglasses prescription. These are abbreviations for Latin terms that mean "right eye" (OD) and "left eye" (OS). Sometimes, you might see a third abbreviation: "OU." This means "both eyes."

Do contact lens prescriptions differ from those for eyeglasses?
Yes. Because contact lenses sit directly on the surface of your eye, they need to have the same curves as your eye.

Last Updated: January 29, 2022