How the Eye Works and Common Vision Problems
The gift of sight is one of our most important senses. Without vision, many daily activities, such as reading and driving, would be difficult. With a good understanding of how the eye works and the importance of total eye care, healthy eyes and good vision can be maintained.
How does vision work?
Vision occurs when light reflected off an object passes into the eye and is interpreted by the brain. The eye is made up of many different parts which work together to provide sight.
Light enters the eye through the cornea, the clear covering of the eye. The curved surface of the cornea bends the light rays to bring the image into focus. The iris, or colored portion of the eye, is a muscle located just behind the cornea which controls the amount of light entering the eye through the pupil. The lens inside the eye fine tunes the focusing of the image by changing shape, depending on whether the object is close up or far away.
After passing through the lens, light rays are focused onto the retina at the back of the eye. The retina consists of light-sensitive nerve tissue called rods and cones. The rods function best in dim light, while the cones function best in daylight conditions and also perceive color. The rods and cones transform the light images into electrical impulses which are sent to the brain by the optic nerve. When the brain interprets the image, vision occurs.
What is 20/20 Vision?
20/20 vision means that vision is normal at a distance of 20 feet. If a person can only see clearly at a distance of 20 feet what the average person can see at 40 feet, he has 20/40 vision. If they can see at 20 feet what the normal person can see at a distance of 15 feet, they have 20/15 vision or better than average vision.
While these figures measure the ability to see straight ahead, it is also necessary to have good peripheral (side) vision, eye coordination, depth perception, color vision and night vision.
What is refractive error?
Many common vision problems are caused by refractive errors. A refractive error is an irregularity in the way light passes through the eye. Normally, light enters the eye and is focused at a single point on the retina. With a refractive error, light rays do not refract or bend properly to achieve a single focus point on the retina. Instead, light rays either focus in front of the retina, behind the retina, or do not focus at a single point. Refractive errors such as nearsightedness, farsightedness, astigmatism and presbyopia usually result from a problem with the shape of the eye, cornea or lens.
What is nearsightedness?
Nearsightedness or myopia is a condition in which close objects are seen clearly but distant objects appear blurred. Nearsightedness results from a steeply curved cornea or elongated eye.
The increased curvature of the cornea or length of the eye makes it impossible for the lens to change its shape sufficiently to focus the light from a distant object onto the retina. As a result, when looking at something in the distance, light rays are brought to focus just in front of the retina and vision is blurred.
Nearsightedness is most commonly a hereditary condition which usually develops in children around the age of eight or nine. Myopia tends to worsen during its early stages then stabilize during the early adult years. Eye classes, contacts and refractive treatments or surgery are used to correct myopia.
What is farsightedness?
Farsightedness or hyperopia is a condition in which objects at far distances are seen more clearly than those that are near. With farsightedness, the curvature of the cornea is too flat or the eyeball is shorter than normal and the eye does not have enough focusing power to see things clearly. Some focusing effort is needed to see distant objects clearly, but an even greater effort is needed to see near objects clearly.
Farsightedness is usually hereditary and is corrected with eye glasses, contacts and refractive surgery.
What is astigmatism?
Astigmatism is a condition caused by a misshaped cornea. Because the curve of the cornea is uneven, light rays bend improperly and are not refracted equally in all directions. As a result, one focus point on the retina is not achieved and distorted vision occurs. With astigmatism, objects appear somewhat indistinct and distorted rather than sharp and clear.
Astigmatism can occur along with both nearsightedness and farsightedness. It is usually hereditary and can be corrected with glasses, contacts or refractive surgery.
What is presbyopia?
Presbyopia is a condition in which the normally soft, flexible lens shrinks with age and becomes hard and resistant to changes in shape. As a result, the eye has difficulty focusing when looking at close objects and reading vision becomes blurred and difficult.
Presbyopia normally occurs about the age of 40. There is no way to prevent presbyopia, as this loss of focusing ability is a normal part of the aging process. Bifocals, reading glasses and contacts, which aid the eye in focusing close objects and refractive surgery are used to treat presbyopia.
Prevention is the best medicine
Regular eye examinations are an important part of total eye care. Eye exams are necessary not only to measure vision and determine if any correction is necessary, but also to monitor the health of the eye. With early detection and treatment, vision loss from many eye diseases can be prevented. If you are experiencing a vision problem, you should obtain a complete eye examination.