HUMAN EYE

An overview of how the many parts of the eye work together to produce clear vision.

Key points

• Visual acuity (VA) is defined as the clarity of the image seen by the eye. Visual acuity is measured using an eye chart at a distance of 20 feet away.
• 20/20 vision does not mean perfect vision. It is normal, average vision.
• Visual field (VF) is a term used to describe how far you can see to the side. A normal visual field is 180 degrees, which is a half-circle.
• The term "legally blind" means different things in different provinces and states. Ask your optometrist or ophthalmologist what "legally blind" means in your province or state. This way, you can find out if your child is able to drive, is eligible for an assistive devices program or other benefits.

The anatomy of the eye

The eye has many parts that must work together to produce clear vision:

• The sclera, or white part of the eye, protects the eyeball.
• The pupil, or black dot at the centre of the eye, is an opening through which light can enter the eye.
• The iris, or coloured part of the eye, surrounds the pupil. It controls how much light enters the eye by changing the size of the pupil.
• The cornea, a clear window at the front of the eye, covers the iris and the pupil.
• A clear lens, located behind the pupil, acts like a camera lens by focusing light onto the retina at the back of the eye.
• The retina is a light-sensitive inner lining at the back of the eye. Ten different layers of cells work together in the retina to detect light and turn it into electrical impulses.

Anatomy of the eye: 3/4 view

Illustration shows the right eyeball with a section removed to see the interior structures.

The retina

Special cells called cones and rods are located in the retina. These cells are known as photoreceptors and help absorb light.

Cones

The majority of the cones are located in the macula, or central area, of the retina. Cone cells help us see colour and detail. Similarly, the macula allows us to read and clearly recognize people's facial details, such as eye colour and whether they have freckles.

Rods

The majority of the rods are located in the peripheral, or outer area, of the retina. Rod cells allow us to see in poor lighting and give us our night vision.

How the eye sees

For people with normally functioning eyes, the following sequence takes place:

1. Light reflects off the object we are looking at.
2. Light rays enter the eye through the cornea at the front of the eye.
3. The light passes through a watery fluid (aqueous humor), and enters the pupil to reach the lens.
4. The lens can change in thickness to bend the light, which will focus it onto the retina at the back of the eye.
5. On the way to the retina, the light passes through a thick, clear fluid called a vitreous humor. The vitreous humor fills the eyeball and helps maintain its round shape.
6. The light then reaches the back of the eye and hits the retina. The retina translates the light into electrical impulses which are then carried to the brain by the optic nerve.
7. Finally, the visual cortex (or centre) of the brain interprets these impulses as what we see.

What normal vision is like

To understand the vision of someone with an eye condition, it can be helpful to know what normal vision is like.

Imagine a scenario where two people are sitting on the couch in front of you. If you look directly at Person A, you are able to use your macula, or central vision, to see the details of their head and face. Maybe they have freckles, brown eyes and black hair.

At the same time, you are aware that Person B is sitting on the couch beside Person A. However, you are not able to see the same amount of detail on their face. For example, you may only see dark areas where their eyes are. To see Person B, you are using the rest of your retina or peripheral vision. Seeing clearly and sharply in the centre, and blurry in the periphery is considered normal vision.

Vision problems

A problem with any part of the eye can cause vision problems. There are many types of eye conditions that can affect vision in different ways. In some cases, the lens does not focus correctly, or the shape of the eyeball is not round, so the image will appear blurry. This can often be corrected with prescription glasses or contact lenses. When an image is focused behind the retina, this is referred to as far-sightedness. When an image is focused in front of the retina, this is referred to as near-sightedness (myopia).

Some eye conditions affect the retina. For some people, only their peripheral vision is affected, which can cause tunnel vision. For others, only their central vision is affected, which can lead to the formation of blind spots (scotomas). Finally, other eye conditions that may cause vision problems include cloudiness in the lens (cataract), increased eye pressure (glaucoma), damage to the cornea, or problems with the eye muscles.