When the normal eye and visual system function well, this condition is called emmetropia (em-e-TROH-pee-ah). Unfortunately, this is not often the case. About 50 percent of the adults in the United States have difficulty seeing clearly at distance and about 60 percent have difficulty seeing close up with no corrective lenses. One of the more common problems is called a "refractive error" which is the mis-focusing of the light as it is directed onto the retina. Here is some basic information on the most common visual conditions affecting computer users:

Myopia
Nearsightedness, also called myopia, means having good near vision but poor distance vision. For the myopic person, an image at least twenty feet away falls in front of the retina and looks blurred. Nearsightedness results when an eye is too long, when the cornea is too steeply curved, when the eyes lens is unable to relax enough to provide accurate distance vision, or from some combination of these and other factors.

Hyperopia
Farsightedness, also called hyperopia, is not exactly the opposite of myopia. For the hyperopic person, an object that is twenty feet or more away is directed past the retina, so that it looks blurred because it hasn't yet focused. Farsightedness results when an eye is too short or the cornea too flat, or from some combination of these and other factors. The main difference between these two conditions is that the eye can increase its focal power (to some degree) to compensate for farsightedness, where it can't reduce it's power to compensate for nearsightedness.

Astigmatism
Theoretically, the surface of the cornea should be almost spherical in shape, like the surface of a ball, so that when light passes through it, it can be focused at a single point. However, nature isn't always perfect, and the cornea is often "warped" so that it more closely resembles a barrel than a ball. The lens can also be irregular in shape. These distortions can be significant enough so that the light that passes through the cornea and lens in the vertical orientation will focus at a different spot from the light that passes through in the horizontal orientation. These two points of focus will cause a blur, or astigmatism (a-STIG-ma-tism), between them.

If the difference between these two points of focus is great enough, the eye will strain and headaches, blurring of vision, or tiring may result. Astigmatism in small amounts is very common and not of great concern. However, about 23 million Americans have a significant amount of astigmatism that requires correction. Glasses correct astigmatism, but do not change the amount of astigmatism; that is, they will not "cure" the problem.

Presbyopia
Presbyopia usually becomes apparent after the age of 40. The presbyopic person must hold their reading material at a farther distance away if they hope to see it clearly. One advantage of using a VDT in the workplace is that it is farther away from the eye than paper reading material. For the pre-presbyopic viewer who does not use reading glasses, this could delay symptoms. However, once reading glasses are prescribed, caution must be taken to assure that the entire viewing area is available.

Binocular Vision
Seeing a clear 20/20 is a good indication that your eyes might be doing a good job. However, sharp eyesight is just one of the functions that your eyes perform. Since we have two eyes, we must make sure that they are working in harmony with each other. One of the most fascinating abilities of the visual system is to take images from two eyes and put them together into just one picture. You don't normally see two images, so the idea might sound strange, but double vision can occur and is one of the most dangerous manifestations of vision problems. Imagine seeing two cars coming at you as you drive down the road!

Here's how the brain keeps us from going off the road. Let's assume that you have two eyes and they are both working about equally well. As you look at just one object, each eye receives an image of that object. Both of these images are transmitted back to your brain, but they are then 'fused' together by the brain into one image. In order for that to happen, both eyes must be pointed at the same object, and the images have to be approximately equal in size and clarity.

Now, if one eye does not aim at the same spot as the other, each eye will be 'looking' at a different object and the two images won't match up. When the images are transmitted back to your brain, they will stimulate two different groups of brain cells, and you will experience two images: seeing double. After a short time, your brain will decide to turn off, or suppress, the picture from the eye that is pointed in the wrong direction so that you can see one image again.

This suppression is necessary for our visual survival, but it is not the way we were made to see. This suppression of an image is the brain's way to make our daily tasks easy and comfortable in stressful situations. Thus, you might think that suppression of an image is devastating but it actually works pretty well. What is more serious, however, is when there is a 'competition' between the eyes and they are struggling to work together. This problem is much more common than the suppression problem. It is this competition that causes the person to struggle with reading tasks and can lead to poor reading comprehension and job performance. Adequate binocular function is important to successful work related tasks.