Changes in vision are a normal part of the aging process. This section of our web site discusses the aging eye, alerts you to abnormal changes in vision, and discusses the primary causes of vision loss for adults, including cataract, glaucoma, macular degeneration, and diabetic retinopathy.

At VisionAWARE, you can discover the range of services for people with low vision, learn about optical and non-optical devices to help you continue your daily activities, find self-help groups and self-study options, and explore our many links that can help you find services in your home area.

If you have experienced recent moderate to severe vision loss, we hope that VisionAWARE will give you a greater understanding of your vision impairment and the range of vision rehabilitation services that are available to you.

About the Eye and How It Works

An overview of the parts of the eye can help you better understand vision changes, vision loss, and eye disorders and diseases, such as cataracts, glaucoma, macular degeneration, and diabetic retinopathy.

Diagram of the Eye, viewed from the side, as a cut apple

To understand this diagram of the eye, try to picture it as being split in two, like an apple that’s been cut in half. Imagine yourself looking into the eye from the cut side.

The Cornea

The cornea is a transparent dome-shaped membrane that covers the front part of your eye. It functions as a window and allows light to enter your eye through the pupil and the iris. It also helps to focus the light rays that help you see words and images clearly.

The cornea does not contain any blood vessels, but instead contains nerve endings that make it extremely sensitive. That is why a scratch or a loose eyelash is so painful.

Aqueous Humor

Aqueous humor is a clear, watery fluid contained in a chamber behind the cornea that helps control the pressure within your eye. It flows between – and nourishes – the cornea and the lens.

The Sclera

The sclera is a tough white outer coating of fibrous tissue that covers your entire eyeball (all the way around!) except for the cornea. The muscles that move the eye are attached to the sclera. The name sclera comes from the Greek word “skleros,” which means “hard.”

The Iris and the Pupil

The iris is a muscle that surrounds – and encircles – the opening in the center of your iris, called the pupil. The iris regulates the amount of light that enters your eye by adjusting the size of the pupil opening.

In bright light, the iris closes (or constricts) and makes the pupil opening smaller to restrict the amount of light that enters your eye.

The iris in bright light

In dim light, the iris opens (or dilates) and makes the pupil opening larger to increase the amount of light that enters your eye:

The iris in dim light

In addition, it is the iris that determines your eye color. People with brown eyes have heavily pigmented irises, while people with blue or lighter-colored eyes have irises with less pigment.

Therefore, people with lighter-colored eyes should wear sunglasses outdoors, especially during the summer. According to Prevent Blindness America, extended exposure to ultraviolet (UV) light (such as sunlight) has been linked with cataracts and macular degeneration.

The Lens

The lens is composed of transparent, flexible tissue and is located directly behind the iris and the pupil. It is the second part of your eye, after the cornea, that helps to focus light and images on your retina.

Because the lens is flexible and elastic, it can change its curved shape to focus on objects and people that are either nearby or at a distance.

The ciliary muscles, which are part of the ciliary body, are attached to the lens and contract or release to change the lens shape and curvature.

The lens becomes more rounded to focus on near objects (see Fig. 1):

Fig. 1: More rounded lens can focus on near objects

More elongated (or stretched) to focus on objects that are far away (see Fig. 2):

Fig. 2: More elongated lens can focus on far objects

The Choroid

The choroid is a dark brown membrane that is rich with blood vessels, located between the sclera and the retina. It supplies blood to the retina and nourishes all of the other structures within the eye.

Contained inside the choroid layer is the vitreous humor, a transparent jelly-like substance that fills the interior of the eyeball (the orange area in the eye diagram) and gives the eye nourishment and shape.

The Retina and Optic Nerve

The retina is the light-sensitive membrane that lines the inside surface of the eye. Nerve cells in the retina convert incoming light into electrical impulses. These electrical impulses are carried by the optic nerve (like your television cable) to the brain, which finally interprets them as visual images.

The macula is the small sensitive area in the center of the retina that provides clear central vision. The fovea is located in the center of the macula and provides the sharpest detail vision.

The Prevent Blindness web site provides a view of the inside of the eye and its component parts at "How We See."

You can get a close up look at different parts of eye in this eye diagram from the National Institutes of Health.

What are myopia, hyperopia, astigmatism, and presbyopia?

Myopia (nearsightedness)

With myopia, you can see close-up objects clearly, but distant objects are blurry. Myopia occurs when the eyeball is too long for light rays to focus correctly on your retina. The more myopic you are, the blurrier your vision will be at a distance and objects will have to be closer to you in order to see them clearly.

Hyperopia (farsightedness)

With hyperopia, you can see distant objects clearly, but close-up objects are blurry. Hyperopia occurs when the eyeball is too short for light rays to focus correctly on your retina.

Astigmatism

If you have an astigmatism, the surface of your eye (the cornea) is not perfectly round; instead, it is more oval-shaped, which doesn't allow light rays to focus clearly on a single point on your retina. Astigmatism is usually accompanied by myopia or hyperopia.

Presbyopia

With presbyopia, you'll have difficulty focusing on close-up words or images. Most people are between 40 and 50 when they realize for the first time that the letters of the telephone book are "too small" or that it's necessary to hold the newspaper further away in order to see clearly. At the same time, the ability to focus on objects that are far way remains normal.

Here is what you see with normal vision:

Here is what you see if you have a refractive error. The image appears blurry at a distance if you have myopia or appears blurry close-up if you have hyperopia or presbyopia.