Eye Care Dictionary


Normal vision, where parallel light rays enter the eye and converge perfectly on the retina.


In some patients there may be an irregular shape with either, or both, the cornea or the lens of the eye. This causes distorted vision because as parallel light rays enter the eye, they don’t converge on the same focal point. Images can appear distorted or stretched out. This tends to be especially obvious with night vision.


Lenses are measured in diopters. For engineering minds, it is equal to the reciprocal of the focal length measured in meters. If you have a 2 diopter lens, parallel rays entering the lens would come to a point 0.5 meters behind the lens.


In people with hyperopia, when parallel light rays enter the eye they converge behind the retina, instead of directly on it. This causes blur that is often worse up close then far away.

Focusing or Accommodation

The ability for the eyes to focus on an object at which they are aimed and easily shift focus from one object to another. This allows you to shift your attention from a book to a person walking towards you and back. Sustained focus affects the ability to read or write for longer periods of time.


In people with myopia, when parallel light rays enter the eye they converge in front of the retina, instead of directly on it. This causes blur that is worse at a distance then up close. Basically the eyeball is too long for the normal focusing power of the eye.


As the lens inside the eye loses it’s elasticity, the eye loses its ability to focus. Can begin as early as 35 or as late as 45, but it progresses until the lens loses all flexibility, the vision then stabilizes, but bi- or tri-focals become necessary to see in all distances.

20/20 Vision

In layman’s terms, if you have 20/20 vision, you’re seeing what the average individual with emmetropia sees at 20 feet. The first number remains a constant and as the second number increases such as 20/50 it indicates that you need to be at 20 feet to see what the average person can see at 50 feet and so forth.

Legal Blindness

20/400 vision is considered legal blindness. A severely reduced visual field can still be defined as legal blindness, even if 20/20 vision is possible.


A general term describing a group of eye diseases characterized by higher than normal intraocular pressure. This high pressure causes damage to the nerve fiber layer of the retina, the optic disk and ultimately results in partial or complete vision loss. Glaucoma is one of the leading causes of permanent vision loss in the United States. (

Intraocular Pressure

Inside the eye we have a flow of aqueous humor. It’s constantly being produced and drained from the eye. If there’s a problem in the production of aqueous, where too much is being made, or a problem at the drain, where it’s not draining as it should, the pressure in the eye will increase. Damage to the drain can occur after a traumatic eye injury, or from the physiology of the drain as determined by genetics.

Aqueous Humor

Watery fluid between the lens and the cornea of the eye, which can bring nutrients to the internal tissues of our eye.

Macular Degeneration

The leading cause of irreversible severe vision loss in the western world. Macular degeneration is typically broken down into two types, commonly called dry macular degeneration and wet macular degeneration. (

Dry Macular Degeneration

Also known as atrophic, or non-exudative macular degeneration, is slowly progressive and the most common type of macular degeneration.

Wet Macular Degeneration

Also known as Exudative macular degeneration, is rapid severe vision loss, sometimes taking less than a couple days. The retinal pigment epithelium (a layer of the retina) detaches and the choroid, the layer beneath the retina, experiences neovascularization (new blood vessel growth, often leaky blood vessels).

Computer Vision Syndrome

Can occur with habitual computer use of just 2 hours a day. Symptoms can include headaches, burning dry eyes, difficulty focusing, fatigue, eyestrain, eye aches, double vision, blurry vision, light sensitivity and pain in your neck and shoulders. Treatment is computer eyeglasses, sometimes accompanied by artificial tears as needed.


Watery eyes.


Loss or lack of development of clear vision in only one eye. The cause is the lack of use of that eye in early childhood. If an eye never sees clearly during development it may never see clearly no matter what corrective lens is placed in front of the eye later in life. It’s important during development for the brain to receive clear images as it builds the connections between the eye and the brain through the optic nerve.

Color Deficiency

The inability to distinguish certain shades of colors, or in more severe cases, see colors at all. Color deficiencies are almost always hereditary and affect one in twelve boys, but only one in two hundred girls. Convergence Insufficiency-is an eye coordination problem where the eyes have a tendency to drift outward when reading or doing close work.


Also known as an eye turn. It is sometimes referred to as crossed eyes or a lazy eye. Poor eye muscle control usually causes this.

Looking for more definitions?

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