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Introduction: Optics and Refractive Error

Nearsightedness, Farsightedness, Astigmatism and Presbyopia

This page discussed several topics concerning "refractive error," or the need for glasses or contact lenses. Information on "eyestrain" and visual problems related to computer use are covered as well. You can use the cursor to point and click one of of the graphics to see its discussion.

 

Conditions Discussed on this Page:

How the eye focuses light
The need for optical correction for distance vision
Emmetropia
Nearsightedness (myopia)
Farsightedness (hyperopia)
Astigmatism
Presbyopia
Anisometropia
The need for optical correction for near vision (reading glasses)
How vision is tested
What is Eyestrain?
How to read and understand a glasses or contact lens prescription

Go to the Vision Testpage for a printable eye chart, Amsler grid, and a color vision test.

Go to  the Simulation of Visual Disorders page for simulations of refractive errors as well as other eye disorders such as cataract and glaucoma.

For eye anatomy explanations, go to ANATOMY

How the eye focuses light

In order for vision to be clear, the eye must focus light onto a precise spot on the retina. This spot is called the macula, and is located straight back through the eye on the inside back surface of the eye. This has been compared to the film of a camera. When light first encounters the eye, the cornea is the first surface that is reached. The simple curvature of the cornea accounts for about 80% of the focusing that the eye does. Light then passes through the pupil and comes to the lens of the eye. The lens does the rest of the focusing. The lens is also able to change the amount of focusing that is does, so things at different distances can come into focus (like an auto-focusing camera). The closer that an object is to the eye, the more focusing the lens has to do in order to make the image clear.

The need for optical correction for distance vision:

Emmetropia (normal sightedness)

Emmetropia really just means normal sightedness. That is, the eye has 20/20 vision without any correction. That is not like hyperopia where you can see 20/20 without correction as long as your muscles are able to self-correct for the hyperopia. As you age, hyperopia will eventually require a correction for distance, while emmetropia will never require any correction for distance.

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Nearsightedness (myopia)

With nearsightedness or "myopia", light is focused in front of the retina rather than directly on it. This leads to a naturally closer point of focus. Depending on how nearsighted the eye is, a close object comes into focus without the lens in the eye having to work to bring it into focus. Unfortunately, the lens in the eye cannot "defocus", so the distance vision will always be blurry (without optical correction) for a nearsighted eye.

There are several reasons why an eye may be nearsighted. If the curvature of the cornea is too steep, the light will be focused in front of the retina. Some eyes grow abnormally long, which can lead to very high levels of nearsightedness. Some types of cataracts will cause the lens to focus light more strongly, leading to increasing nearsightedness.

Nearsightedness is corrected optically with a minus powered lens (glasses or contact). Refractive surgery such as LASIK can more permanently correct nearsightedness by flattening the cornea. Ortho-K can flatten the cornea with rigid gas permable contacts that you wear only while you are asleep. The effect lasts as long as you wear the contacts and surgery is not necessary.

Farsightedness

Farsightedness (or hyperopia) is a somewhat misunderstood term. It implies that the distance vision would be clear, but the near vision would be blurry (the opposite of nearsightedness). However, this is not necessarily true. With farsightedness, the eye does not focus light strongly enough to reach the retina. Instead, light is focused behind the retina. But for many people, the lens in the eye is capable of adding extra focusing (usually for focusing on near objects such as reading). Thus, if an eye has enough focusing ability, it can focus away farsightedness, and the distance vision will be clear without glasses. However, this can take away from its ability to then focus on a near object.

There is a natural decline in the ability of the lens to focus as one ages. Someone may be unknowingly farsighted and have clear distance vision at age 30. However, by age 50, the lens in the eye can no longer focus well, and the person may need glasses for distance vision. There is no refractive surgery procedure which can reliably correct farsightedness.

Astigmatism

Astigmatism occurs when the curvature of the cornea is not perfectly round in all directions. In one direction (or axis) the curvature is greater (steeper), and in the opposite direction is is lesser (flatter). This can be compared to the curvature of a spoon. In a round soup spoon, all of the curvatures are the same, and there would be no astigmatism. However, in a teaspoon, the spoon is curved more gradually along the length of the spoon, and more steeply along the width of the spoon. The direction of astigmatism is measured in degrees from 1 to 180 (like degrees on a protractor). 180 degrees is perfectly horizontal, while 90 degrees is straight up and down. Glasses correcting astigmatism add extra power in the direction needed to equalize the difference in curvature of the cornea.

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Presbyopia

Presbyopia is loss of the lens' ability to change shape to focus on near objects due to aging. Typically, presbyopia becomes noticeable by the time a person reaches the early or mid 40s. A convex (plus) lens is used for correction when viewing near objects. These lenses may be supplied as separate glasses or built into a lens as bifocals or variable focus lenses. In the graphic above, you can see the difference between focussing at distance with the red straight lines of light compared to the green diverging lines of light. This is why presbyopia simulates hyperopia for near as the rays tend to focus behind the eye. See graphic.

Anisometropia

Anisometropia is a significant difference between the refractive errors of the two eyes (usually > 3 diopters). When corrected with eyeglasses, a difference in image size (aniseikonia) is produced that can lead to difficulties with fusion of the two differently sized images and even to suppression of one of the images.

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The need for optical correction for near vision:

The focusing ability of the lens of the eye allows objects at different distances to come into clarity with little or no conscious effort on our part. However, this focusing ability definitely declines with age. This decline in focusing ability is called "presbyopia." Presbyopia can occur in spite of any underlying nearsightedness, farsightedness, or astigmatism. The focusing power of the lens in measured in "diopters." To focus on close objects, the lens has to focus increasingly more. The diopter power of the lens is age dependent. It is not fully understood why the focusing ability of the eye declines with age. The following table shows how the power of the lens changes with age:

• A 10 year old has 14 diopters of power
• A 20 year old has 10 diopters
• A 30 year old has 7 diopters
• A 40 year old has 4.5 diopters
• A 45 year old has 3.5 diopters
• A 50 year old has 2.5 diopters
• A 60 year old has 1 diopter

To read at the normal distance of 16 inches, the lens in the eye has to focus about 3 diopters. Note that the lens is only comfortable focusing at its full ability for a short period of time. It is more comfortable focusing with about 1/2 of its full ability for any duration of time. Using the above table, a 40 year old with only 4.5 diopters of power available, may begin to experience difficulty reading at 16 inches for any extended amount of time. A 45 to 50 year old may not be able to focus at that distance at all. It is possible to hold reading material further away than 16 inches, but eventually people will experience that their arms not long enough to read!

Reading glasses offer extra power so that the lens in the eye does not have to excessively focus. If no distance glasses are needed:

• A 40 to 44 year old needs a reading glasses power of about +0.75 to +1.00.
• A 45 to 47 year old may need a +1.25 reading glasses power.
• A 48 to 50 year old may need a +1.50 to +2.00 in reading glasses.
• A person older than 51 may need a +2.25 to +2.50.

The exact power preferred for reading glasses may vary based on what distance an individual prefers to hold the reading material. If a person also has a distance glasses prescription, the reading power needed is added to that distance prescription. In this situation, bifocals can be used. A bifocal lens has the distance prescription at the top of the lens, and the extra power needed for reading at the bottom.

A reduced range of focus occurs as the decline in focusing worsens . A person may be able to use reading glasses to read at 16 inches, but not at 12 inches, and not at 19 inches. The use of "progressive" bifocal lenses allow for an increased range of focus, with there being an increasing power in the bifocal from top to bottom (also a "lineless" bifocal). A trifocal has a weaker powered section in the center for reading at arm's length, and a more powerful section at the bottom for closer reading. There are also contact lenses with bifocal powers.

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How vision is tested:

A person's "uncorrected" vision refers to the visual acuity when no glasses or contact lenses are used. The "best corrected" vision is the visual acuity with the best glasses or contact lens prescription for that person. Each eye is usually tested separately, although the vision may be slightly better with both eyes together.

The notation of visual acuity is written as a fraction, with normal vision being 20/20 (twenty twenty vision). At a 20 foot distance, (the top number in the fraction, or testing distance), a person with normal vision should be able to read the small 20/20 line on an eye chart. The smallest line that an eye can read is its visual acuity. If larger lines than the 20/20 line are all that can be read, the visual acuity may be 20/30, 20/60, etc. The larger the second number is, the worse is the vision. A person with 20/200 vision would have to come up to 20 feet to see a letter that a person with normal vision could see at 200 feet! Similarly, if the vision is 20/10, it means that the vision is better than normal. A person with 20/10 vision can read a letter at 20 feet that a person with normal vision would have to come up to 10 feet to read.

Eye charts in offices are calibrated for different test distances, so that rooms do not have to be 20 feet long.

For a printable eye chart, go the VISION TEST PAGE.

Certain visual acuities have special significance. Some of these are:

• 20/20 vision is considered normal vision.
• 20/40 vision uncorrected in at least one eye is the vision required to pass many state driving tests (for driving without glasses).
• 20/50 vision or worse is often the visual reduction that is considered bad enough by most patients to need cataract surgery, if that is the cause of the visual loss.
• 20/200 vision is considered legally blind (a person is not considered legally blind unless both eyes are 20/200 or worse, or if there are severe constrictions in the peripheral vision).

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What is eyestrain?

Eyestrain refers to a sensation of fatigue of the eyes. Usually this is associated with prolonged reading or near work. A person experiencing eyestrain may have some or all of these symptoms:

• Eye fatigue and even mild aching
• Ocular redness
• Blurred vision at near or distance
• Trouble holding things in focus, with fluctuation of vision
• A slower than normal adjustment when focusing between near and distance  
• Headache (usually over or behind the eyes)
• A sensation of ocular dryness
• Brief double vision

There are several different things that can lead to eyestrain symptoms. When the muscle inside of the eye that controls focusing is overworked, symptoms can occur. In many cases, these symptoms will not start immediately, but only after several hours of work. When the muscle in the eye becomes fatigued, the eyes may feel uncomfortable or ache. The vision may blur off and on. A mild ache can occur if the eyes continue to work. In some cases, the muscle within the eye can become so fatigued that it cannot fully unfocus, leading to blurred distance vision. The following things can contribute to eyestrain:

• Having to read or use a computer at a fixed, set distance for a long continuous period of time. Even if a person has more than adequate focusing ability, focusing at a set distance continuously can fatigue the lens.

• Having to read or work at very close distances. This requires much more focusing and leads to more rapid fatigue.

• Using inadequately powered reading glasses, or using an outdated glasses prescription.

• Working in situations with inadequate lighting, or with glare from overhead lighting.

• Having other underlying eye problems, such as ocular allergy or dry eye.

• Having an imbalance in eye muscle alignment, so that the eyes have to fight to stay fixated on a near object.

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Things that can be done to treat or prevent eyestrain include:

Take frequent, short breaks from near work, at least every 20 minutes, for 1 or 2 minutes. During this time, close the eyes or look off into the distance. This helps the eye muscles to stay refreshed.
Change the distance that you work frequently. If the eyes are feeling increasingly fatigued, hold things further away rather than closer to you. Avoid getting very close to what you are reading.
Get an eye exam to determine if the distance glasses prescription if correct, and if you may need reading glasses.
Sometimes reading glasses need to only be used near the end of the day when the eyes are becoming fatigued. Other people may need separate reading glasses to use at work or with a computer.
Treat any other eye problems, such as dry eye, as indicated. Artificial tears used occasionally may help.
Try to improve the lighting situation, if possible. If reading, having the light source coming from behind, over your shoulder, helps to prevent glare problems. If using a computer, dark print on a white or light gray background is less fatiguing to the eyes than multicolored print. Sometimes a glass filter over a computer screen with an anti-reflective coating can help with computer glare. These can be purchased from office supply stores. Also, do not forget to frequently dust the computer screen, since this can blur the image on the screen.

Eyestrain will not permanently damage the eyes or cause a loss of vision. However, it can be very uncomfortable and lead to a loss of productivity.

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How to read and understand a glasses or contact prescription:

Glasses prescriptions are not difficult to understand. First of all, the right eye is usually listed first, and is noted by O.D. The left eye is O.S. The prescription has mainly three parts: the sphere, the cylinder and axis, and the add.
In this example, the bifocal is shown with three patient choices: traditional bifocal or trifocal and a high quality Progressive Addition Lens (no-line bifocal).

Contact lens prescriptions are similar to glasses prescriptions. However, as shown here they also state the name and type of contact lens being used, any tint in the lens, the base curve of the lens, and the diameter. With many brands, the color and diameter are not listed as they only come with standardized specs. The power of contact lenses are usually just plus or minus sphere powers. Only with special astigmatism-correcting toric lenses is there a cylinder power (always a minus power for contacts) and an axis.



The base curve determines how tight the lens is on the eye. It usually ranges from an 8.3 (tight) to a 9.0 (looser). In soft lenses, there are usually only a few choices for the base curve (tight, medium, or loose), and there may be only one diameter (the width of the lens). Hard lenses and gas permeable lenses are custom made and have more choices. This particular prescription shows a toric (astigmatism) lens for the right eye and a spherical lens for the left eye.


The sphere determines nearsightedness or farsightedness. No sphere power is noted as "plano". If the power is a minus, it is a nearsighted prescription. If it is a plus, it is farsighted. Mild prescriptions are in the range of plus or minus 1 to 3, while high prescriptions are over plus or minus 5 to 7.

The cylinder and axis represent the astigmatism correction. Cylinder is power in a certain direction, and can be written as a plus or a minus power (ophthalmologists usually use plus, optometrists and all contact lens specs use minus). The axis is the direction of the power. It is measured in degrees, from 1 to 180. Axis 180 is perfectly horizontal, while axis 90 is straight up and down. This is like the degrees on a protractor. Most people would not notice a change in axis of 5 to 10 degrees, unless the cylinder power is fairly high (say, over +2.00). Many people do not tolerate glasses with high cylinder due to distortion.

The add is additional power placed in a bifocal. This is always a plus power, and is similar to a plus farsighted sphere power. This usually ranges from a +1.00 to +3.50. The average highest power that people need in a bifocal for reading is a +2.25.

Contact lens prescriptions are similar to glasses prescriptions. However, as shown above they also state the name and type of contact lens being used, any tint in the lens, the base curve of the lens, and the diameter. The power of contact lenses are usually just plus or minus sphere powers. Only with special astigmatism-correcting toric lenses is there a cylinder power (always a minus power for contacts) and an axis.

The base curve determines how tight the lens is on the eye. It usually ranges from an 8.3 (tight) to a 9.0 (looser). In soft lenses, there are usually only a few choices for the base curve (tight, medium, or loose), and there may be only one diameter (the width of the lens). Hard lenses and gas permeable lenses are custom made and have more choices.

 

   
 
     
 
myopia emmetropia hyperopia astigmatism presbyopia