How the Eye Works-Common Vision Problems

Wednesday, August 20, 2008

In its simplest sense, your eye is like a camera. Your eye has

  • A reusable "film" at the back, called the Retina
  • A lens system, which includes the transparent covering called the Cornea and a spherical lens
  • A variable opening at the front, called the Pupil
  • Various sets of muscles (The muscles control the size of the opening, the shape of the lens system and the movements of the eye).

On the back of your eye is a complex layer of cells known as the Retina. The Retina reacts to light and conveys that information to the brain. The brain, in turn, translates all that activity into an image. Because the eye is a sphere, the surface of the Retina is curved.

In the Retina, sensory cells called rods and cones change the photons of light into electrical signals, which are then transmitted to and interpreted by the brain. The ability to focus the light on the Retina depends on the shapes of the cornea and the lens, which are controlled by their inherent shapes, their elasticity, the shape of the eyeball and sets of attached muscles.

So, when you look at something, muscles attached to the lens must contract and relax to change the shape of the lens system and keep the object focused on the Retina, even when your eyes move; this is a complex set of muscle movements that is controlled automatically by your nervous system.

When you look at something, three things must happen :

1. The image must be reduced in size to fit onto the Retina.

2. The scattered light must come together that is, it must focus at the surface of the Retina.

3. The image must be curved to match the curve of the Retina.

As shown in the image below, light passes through the cornea and Pupil, is bent (refracted) by the lens, and comes to a point (focus) on the Retina, where the image is formed.

To do all of that, the eye has a lens between the Retina and the Pupil (the "peep hole" in the Center of your eye that allows light into the back of the eye) and a transparent covering, or cornea (the front window). The lens and the cornea work together to focus the image onto the Retina.

Out of Focus

Have you ever looked at a written page and it looked blurry? Perhaps, you have taken an eye exam for your driver's license and found that you couldn't read the signs very well. May be you have discovered that, as you are nearing 40, you have to hold books and newspapers farther away to read them.

Changes in the shape and size of your eyes can cause problems that affect the way you see elements that are close or far from you. These problems are referred to as refractive disorders. So you go to the Ophthalmologist or Optometrist and find that you need glasses. And your life is not the same any more!

Most vision problems occur when the eye cannot focus the image onto the Retina. Here are a few of the most common problems:
  • Myopia (Nearsightedness)
  • Hyperopia (Farsightedness)
  • Astigmation
  • Presbyopia
Myopia (Nearsightedness)

In Nearsightedness (Myopia), the light from distant objects gets focused in front of the Retina rather than on it. Myopia happens usually when the eyeball is too long; however, it is sometimes caused by too much focusing power in the lens system. The result is that the person can see close-up objects clearly, but distant objects are blurry.

Hyperopia (Farsightedness)

In Farsightedness (Hyperopia), the light gets focused in back of the Retina rather than on it. See following illustration. Hyperopia usually happens when the eyeball is too short or when the focusing power of the lens system is too weak. The result is that a person can see distant objects fine, but close-up objects are blurry. Hyperopia can by corrected by using a convex lens to concentrate or converge the light so that when it passes through the lens system, it comes to focus on the Retina.


In Astigmatism, the shape of the Cornea or the lens is distorted so that the light comes into two focal points. Imagine that the lens is egg-shaped instead of spherical and that light coming over the top and bottom edges is brought to a different focal point than light coming over the right and left sides. To correct this problem, a lens that is shaped to correct the distorted shape of the eye's lens system is made.


In Presbyopia, the Cornea and lens of the eye become less stretchy, and therefore cannot change shape as readily to bring light to a focus on the Retina; this happens naturally as we grow older and is usually observed when people reach their 40s. If you have Presbyopia, you have trouble focusing light from near objects on the Retina. To correct this problem, you might get a pair of bifocal lenses to replace your existing glasses. If you don't already wear corrective lenses, you may be able to simply use reading glasses.

LASIK is very effective in treating Myopia and, in many cases, can correct vision problems resulting from Astigmatism and Hyperopia as well. However, Presbyopia is not easily corrected through the use of laser eye surgery.


(Click on the image to view full size)

  © Blogger templates The Professional Template by 2008

Back to TOP