Glossary of terms relating to
eyesight and vision

A - B - C - D - E - F - G - H - I - K - L
M - N - O - P - R - S - T - V - W - Y

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The expression for normal eyesight (or 6/6 in countries where metric measurements are used). This notation is expressed as a fraction. The numerator (1st number) refers to the distance you were from the test chart, which is usually 20 feet (6 meters). The denominator (2nd number) denotes the distance at which a person with normal eyesight could read the line with the smallest letters that you could correctly read. For example, if your visual acuity is 20/100 that means that the line you correctly read at 20 feet could be read by a person with normal vision at 100 feet. The Snellen chart, which consists of letters, numbers, or symbols, is used to test visual acuity (sharpness of eyesight). A refraction test is used to determine the amount of correction needed for a prescription when treating refractive error such as astigmatism, myopia, or hyperopia. See Refraction Test.

AC/A Ratio

Accommodative convergence / accommodative (measured in prism diopters/diopters). The convergence response of an individual (amount the eyes turn inward) in relation to the amount of stimulus of accommodation (eye focusing). The normal ratio is 4:1.

Accommodation (eye focusing)

The eye's ability to adjust its focus by the action of the ciliary muscle, which increases the lens focusing power. When this accommodation skill is working properly, the eye can focus and refocus quickly and effortlessly, which is similar to an automatic focus feature on a camera. The ciliary muscles must contract to adjust for near vision, which causes the eye's crystalline lens, which is flexible, to be squashed. For distant vision, the ciliary muscle must relax and the eye's crystalline lens is stretched out. The ability of the eye to accommodate does decrease with age due to the crystalline lens becoming less flexible causing a condition called presbyopia. (See "Presbyopia").

Accommodative Fatigue

This clinical condition is also called Ill-Sustained Accommodation. It is the inability of the eye to adequately sustain sufficient focusing over an extended time period. The most common sign or symptom is blurred vision after prolonged near work such as reading and using a computer. In addition, such patients often have asthenopia (eyestrain), general fatigue, headaches and nausea, excess tearing, and an unusual sensitivity to light. Clinical signs include: normal amplitude of accommodation, decreased PRA, and the patient generally fails the +/-2.00 D flipper test. Plus lenses (glasses or contacts) and vision therapy are effective in treating this condition.

Accommodative Esotropia (clinical condition)

When an individual is focusing on a near object and his or her eyes are turning inward too much. It is caused by either uncorrected hyperopic refractive error and/or a high accommodative convergence/accommodation (AC/A) ratio. The average age of onset is 2 1/2 years. It is most noticeable when the child is tired or sick. This is treated with plus lenses (glasses or contacts) to help straighten the eyes. In some cases, vision therapy and corrective lenses are prescribed. (Please note that Accommodative Esophoria is a condition similar to accommodative esotropia but lesser in extent.)

Accommodative Excess (AE)

This clinical condition is also called accommodative spasm. It is an over focusing, over stimulation of the focusing action of the crystalline lens causing an inability to relax the focusing system which may result in blurry vision when focusing at distance objects. Other symptoms include holding near work closer than normal, headaches with near work (such as reading or using a computer), eyestrain associated with near work, and possible double vision. Clinical signs include: patient accepts more minus on accommodative rock but blurs with plus lenses, lower NRA than PRA, dynamic retinoscopy findings indication of over accommodation and/or slow relaxation of accommodation, and reduced or erratic distance visual acuity. Treatment includes a low plus lens and/or vision therapy. To see the American Optometric Association's guidelines for vision therapy, please click here. (See "Pseudomyopia")

Accommodative Infacility

A clinical condition in which the individual has difficulty changing eye focus from distance to near. Symptoms include eyestrain associated with near work (such as reading or using a computer), periodic blurring of distance vision especially following sustained near visual work, tendency to hold near work closer than expected, headaches with near work, and possible double vision. Clinical signs include: patient will have difficulty with both the plus and the minus lens (fails +/- 2.00 D flipper test), low PRA and NRA, and poor recoveries on Bell Retinoscopy. Vision therapy is an effective treatment option.

Accommodative Insufficiency (AI)

This clinical condition is also called non-presbyopic accommodative insufficiency. It is an under focusing, a lack of focusing ability at a near distance. Symptoms include eyestrain, blurred vision, occasional or constant when doing near work (such as reading or using a computer), occasional unusual sensitivity to light, excess tearing, headaches, and general fatigue. Clinical signs include: patient will have difficulty with a minus lens, low amplitude of accommodation, low PRA and higher NRA. Vision therapy like Eyerobics is an effective treatment option.

Accommodative Vergence

a convergence response (to turn the eyes inward) which occurs as a direct result of accommodation (eye focusing). (See "Vergence")

Acuity- clearness of eyesight

Depends on the sharpness of images and the sensitivity of nerve elements in the retina. (See "Near Acuity" and "Distance Acuity")


Prescription strength of a plus lens which is used for near vision. A plus lens can be added to another lens such as a minus lens for distance vision. (See "Bifocal Glasses" and "Presbyopia")


The eye's ability to still see an image during eye blinks and even after the viewed object is no longer present. The most common example is seeing light after the flash of a camera.


Pigmentation is deficient or absent. May occur in skin, hair, and eyes. Ocular albinism is a pigmentation deficiency occurring mainly in the eyes. Individuals with albinism including ocular albinism commonly have decreased visual acuity (20/70 -20/200), strabismus, photophobia, and nystagmus. There is no known treatment. Individuals may benefit from low-vision aids. Treatment options for strabismus and nystagmus does apply to these individuals. (See "Strabismus" and "Nystagmus")


Proper fusing (uniting) of images to each eye.

Amblyopia (clinical condition)

Reduced visual acuity (poorer than 20/20) which is not correctable by glasses or contacts and is not caused by structural or pathological anomalies. This condition is often called "lazy eye" because it is typically the result of disuse. It is usually marked by blurred vision in one eye and favoring one eye over the other. About two percent of the population is affected.

Types of functional (reversible) amblyopia:

refractive- anisometropia (the two eyes have different refractive powers), or other amblyopiogenic refractive errors (hyperopia, myopia, or astigmatism)
strabismic - misalignment of the two eyes in which they point in different directions
form deprivation (may also be referred to as amblyopia ex anopsia)- caused by conditions that prevent light from entering the eye. These may include congenital ptosis (droopy eyelid), corneal opacity, or cataract.
Treatment options for functional amblyopia are eye patching, prescription lenses, prisms, and vision therapy.


Any optical error such as hyperopia, myopia, or astigmatism. Also called refractive error.

Amplitude of Accommodation (AA)

A measurement of the eye's ability to focus clearly on objects at near distances. This eye focusing range for a child is usually about 2-3 inches. For a young adult, it is 4-6 inches. The focus range for a 45-year-old adult is about 20 inches. For an 80-year-old adult, it is 60 inches.

Amsler grid

See the Amsler grid page


A difference in the size or shape of two visual images when the images should be the same size and/or shape.


The condition in which the two eyes have different refractive powers.

Anomalous Retinal Correspondence (ARC)

A type of retinal projection, occurring frequently in strabismus, in which the foveae (center of the retina that produces the sharpest eyesight) of the two eyes do not facilitate a common visual direction; the fovea of one eye has the same functional direction with an extrafoveal (non-fovea) area of the other eye.

Aperture Rule

A stick-like instrument used in vision therapy to develop convergence and divergence (eye teaming) skills.


Aphakia is the absence of the lens of the eye, due to surgical removal, perforating wound or ulcer, or congenital anomaly; causes a loss of accommodation, hyperopia, and a deep anterior chamber. Complications include detachment of the vitreous or retina, and glaucoma.

Aphakic people are reported to be able to see ultraviolet wavelengths that are normally excluded by the lens. This may have had an effect on the colours perceived by artist Claude Monet, who had cataract surgery in 1923.


Weakness or easy fatigue of the eye due to strain of the ciliary muscle, with pain in the eyes, headache, dimness of vision,etc. accommodative asthenopia - asthenopia due to strain of the ciliary muscle.
muscular asthenopia - asthenopia due to weakness of external ocular muscles.

Astigmatic dialastigmatic dial

The astigmatic dial is a diagram of radiating lines, used to test for astigmatism.

The simple, precise target ends the need for "flip" cross cylinder testing, and permits direct prescription for astigmatism (out of focus vision frequently associated with near or farsightedness).


Light rays entering the eye do not all meet at the same point (similar to a frayed string), which results in blurred or distorted vision. An abnormally shaped cornea typically causes this condition. Occasionally astigmatism exists in the lens of the eye. This condition is corrected by a cylindrical (thoric) eyeglass or contact lens.

Automated refractor

Also called auto refractor. This method determines the eye's refractive error and the best corrective lenses to be prescribed by using a computerized device that varies its optical power mechanically and prints out the results.


The alignment of the lens' cylindrical part; used for correcting astigmatism. This measurement is given in degrees. The values are typically from 90 degrees to 180 degrees.