Changes in vision are a normal part of the aging process. This section talks about the aging eye and alerts you to abnormal changes in vision and the main causes of vision loss experienced by older adults, such as 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, self-study options, and delve into our many links that show you how to find services in your home area.

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

About the Eye and How It Works

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

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, like this:

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:

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

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.

Because the lens is flexible and elastic, it is able to change its curved shape in order 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:

And more elongated (or stretched) to focus on objects that are far away:

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 gives us clear central vision. The fovea is located in the center of the macula and provides the sharpest detail vision.

For a view of the inside of the eye and its parts, click on the Prevent Blindness web site "How We See".

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