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What Is "20/20" Vision?

Firstly, what do you think of when I say normal vision? If you're like most people the first thing that comes to your mind is 20/20 eyesight. But what does 20/20 eyesight really mean?

In 1862, a man named Snellen devised a testing system to determine people's eyesight. He asked people to stand at 20 feet and through experimentation discovered the smallest sized letter that most people with good eyesight could see. He called this a size 20 letter and the vision was recorded as 20/20. At the time it was thought that this was all you needed in the way of vision. Actually it was a great breakthrough at the time. Now, over 130 years later, we have made quantum leaps in our knowledge of the human visual system. Yet, if you walk into many schools and pediatrician's offices and even some eye doctor's offices the old 20/20 standard is still the only way that vision is tested.

20/20 eyesight tells us how well we can see to drive or to see a blackboard. It tells us if we can see a newspaper or a computer screen. But there are many things 20/20 eyesight will never tell us. It will never tell us if you or your child:

  • sees clearly all day long
  • can focus back and forth to the blackboard and book
  • sees single rather than double
  • can read without getting a headache
  • can follow words on a page without losing his place
  • can read without wanting to fall asleep
  • has healthy eyes

Remember the vision chart that hangs in every eye specialist's examining room? The one with a big "E" at the top? That's the basis for measuring 20/20 vision. You are usually asked to identify the letters at a distance of 20 feet. Then you'll be asked to read lines on the chart with one eye and then the other. If you have 20/20 vision, you see at 20 feet what a person with normal vision would see at 20 feet. If you have 20/40 vision, you see at 20 feet what a person with 20/20 vision would see at 40 feet. And, if you're one of the lucky ones (e.g., 20/15), you can see at 20 feet what others would have to move closer (at 15 feet) to see.

Understanding Your Prescription

Diopters measure the refractive power of your eye. Your doctor uses this measure to identify the amount of refractive error in your eyes. Diopter values range from -14 to +14, depending on how significant the error is. A negative diopter value indicates myopia and a positive diopter value signifies hyperopia. And the larger the diopter value, whether it's a positive number or a negative number, the greater the refractive error. Someone with 20/20 vision will likely have something close to zero diopters. If you've ever received a prescription for glasses, it most likely included diopter readings for each of your eyes. In the end, the most important thing to know is that diopters are a precise way of measuring how much vision correction you need, whether it's through laser eye surgery or glasses and contact lenses.

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More About Your Eyes

Over half the population in the United States has common vision problems. Generally, people with vision problems are nearsighted, farsighted, and/or have an astigmatism.

There's another common condition called presbyopia, a condition that many people develop as they age. Presbyopia causes everything a person sees to be a bit fuzzy, which is why many of us require reading glasses. Unfortunately, there's no surgical remedy for presbyopia, so for now, we will probably continue to need those reading glasses.

Until recently, most people addressed nearsightedness, farsightedness and astigmatism by wearing glasses or contact lenses. But today, people are finding an alternative to glasses or contact lenses.

Everywhere, more and more people - perhaps you - are considering a procedure known as laser eye surgery or LASIK, Laser In-situ Keratomileusis, which means reshaping your cornea, from within, using a laser.

Laser vision correction uses quick bursts of excimer laser beams to reshape the cornea, helping to eliminate the corneal errors or aberrations that change our ability to focus as clearly as we should. When errors are eliminated or reduced, images will focus more directly on your retina - the back of the eye - where they should.

Perhaps the best way to understand vision problems - and how reshaping the cornea can help correct them - is to start with learning some basics about the human eye.

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Normal Vision

What you've always heard is true: the human eye really does work like a camera. In normal vision, light rays coming through the cornea and lens converge and focus perfectly and, finally, focus directly onto the retina, at the back of the eye. The retina sends the "signals" to our brain, which registers them.

Image of a Normal Eye

"Poor vision" or vision problems are primarily caused by refractive errors. These "errors" occur when the cornea is shaped in such a way that the images we see do not focus directly on the retina.

Poor vision caused by refractive errors was treated with glasses, contact lenses, or various refractive correction procedures.

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Common Vision Problems

Nearsightedness (Myopia)
You see close objects more clearly than those at a distance. When you're nearsighted or myopic, images in the distance will seem blurry. Your eyes may be longer than normal or the cornea may be too curved, so images focus in front of the retina. People who are nearsighted see near objects more clearly than distant ones. The nearsighted eye is longer than normal so light rays converge and focus before they reach the back of the eye.

Image of a Nearsighted or Myopic Eye

Farsightedness (Hyperopia)
People who are farsighted see distant objects more clearly; however, all objects may be blurred. The farsighted eye is shorter than normal and light rays do not have enough space to converge and focus. When you are farsighted or hyperopic, images that are near (the words on a page, for example) appear to be more blurry than images in the distance. Your eyes may be too short, or your cornea too flat, so images focus behind the retina.

Image of a Farsighted or Hyperopic Eye

Astigmatism is the inablility to focus clearly at any distance because of an irregular or misshapen cornea. Light rays focus at various points within the eye causing distorted vision. Astigmatism is often combined with nearsightedness and farsightedness. This happens because the front part of your eye (corneal is slightly irregular in shape. Astigmatism, another cause of poor vision, occurs when the eye is "football" shaped. Here, images focus on more than one point in front of, or behind the retina. The result is that all images, whether near or far, may be blurry. In mixed astigmatism, symptoms of myopia or hyperopia are combined, resulting in the overall inability to see images clearly.

Image of an Astigmatic Eye

Presbyopia is a normal, age-related change that occurs as we approach our mid-forties. The lens of the eye becomes less elastic and loses its ability to change focus, making it difficult to see up-close. As eyes age, they will experience presbyopia. A natural part of aging begins to blur your reading and near vision by about age 40-45 and gradually worsens. Presbyopia, a physiological weakening of vision due to natural aging, cannot be corrected by LASIK. As mentioned previously, there are currently no approved surgical procedures that correct this condition.

Now that you know the basics about your eyes, and poor vision, you're better equipped to address your eye problem(s). Come in and we'll see how else we can help in correcting or treating your vision problems. This site is dedicated to assist you in informing you on our capabilities.

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* A note on astigmatism:
You may have heard or been told that astigmatism cannot be treated. Today, with the right technology, it can. The FDA has approved select lasers to correct a broad range of conditions. In some instances, that range includes astigmatism associated with either myopia or hyperopia.

Roll over the words that are in bold to view a definition in the column below.

A full listing of terms is available in the glossary.
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