Our eyes are simple tools. They are designed to simply catch light and send it to the brain. However, the method by which they gather, filter and guide the light, as well as how our brain processes the information received by the eye, makes for the wonder of vision. In our interaction with our environment, there is little to compare with the contribution the eyes play. How we use our eyes and visual system dictates how well we survive in our environment.
Unfortunately, this process of eyesight and vision doesn’t always work as well as it should. This is especially true today since many of our viewing tasks are done at a close working distance. This requires the eyes to maintain an active focusing status, which can cause stress and strain on the eyes and the muscles that control them. This can in turn lead to a deficiency in the way the eyes focus the light so that the images are not clear. The conditions that exist when light doesn’t focus properly in the eye are called refractive errors.
The three types of refractive errors are nearsightedness, farsightedness and astigmatism. Nearsightedness, or myopia (my-OH-pee-ah) occurs when the light entering the eye focuses too soon. 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. Farsightedness, also called hyperopia (hi-per-OH-pee-ah), is not exactly the opposite of myopia. For the hyperopic person, an object that is twenty feet or more away (so that the internal lens is relaxed) 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 they eye can increase its focal power (to some degree) to compensate for farsightedness, where it can’t reduce its power to compensate for nearsightedness.
Theoretically, the front surface of the eye (called 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 too can 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. Now you have two points of focus with a blur between them. This is known as astigmatism (a-STIG-ma-tism).
For the most part, glasses can “correct” or compensate for these conditions. Glasses for myopia will “weaken” the light focusing power so that the light will fall further back into the eye and strike the retina. Glasses for hyperopia will increase the focusing power of the light and make it strike the retina closer toward the front of the eye. For astigmatism, the glasses are ground so that the two major directions of light have the power focused together and at the same point. On a simple level, this is the way glasses allow us to see clearly.
There are several types of lenses available that can correct for hyperopia, myopia or astigmatism to enable us to see at different distances. First there are single vision (or “all purpose”) glasses. These have just one focal power and are designed to refocus the light to the proper point. Next there are bifocals, which are basically a single vision lens with a small segment of a higher power imbedded into the lower portion of the lens. This higher power allows the wearer to see at a near distance without having to use the focusing lens within the eye. The trifocal lens is an extension of this concept with two additional segments imbedded in the lens- each being a different power so that near and intermediate viewing targets can be seen.
A more recent advance is that of “progressive” lenses (also mistakenly called no-line bifocals). This lens has a different power in each spot of the lens that is viewed so that all distances can be seen clearly. From the distance to the near viewing range, there are almost an infinite number of focal points.
The above lenses are considered general-purpose glasses because they are usually the only type of lens a person wears. However, there are also “task-specific” lenses that are prescribed for specific viewing requirements. One example of this is sunglasses-lenses designed for bright viewing situations (darkly tinted). Another task specific lens is the reading lens. These are worn for near-point viewing tasks. If one attempts to look at a distant object while wearing these lenses, everything will appear blurry.
Task-specific lenses can come in several designs as well. The option include single vision, bifocal, trifocal and progressive lenses. One of the more common visual tasks today is that of computer viewing, so there have been several types of “computer-specific” lenses developed. So when someone says, “I use computer glasses,” it doesn’t say much about what type of lens the person is using.
A single vision lens for computer use is just that- a single vision lens of a prescription that will allow the person to see the computer screen clearly. However, other areas of viewing might be blurred, either at a near point or distant point. If one wears a task-specific bifocal lens, the upper portion, which is normally designed for distance viewing, is set for the intermediate distance (monitor distance) and the lower portion is for close work (paper reading). These can be effective but are unsightly (with a visible line) and have a limited range of view.
One of the first task-specific lenses designed specifically for computer use was called the “CRT” lens. This is a trifocal design with lines that go all the way across the lens. The very top portion of the lens is for distance viewing; the middle segment for intermediate viewing, and the bottom for near-point reading. Although quite effective, this lens design is very bulky and the divisions in the lens are very obvious. When looking from one portion to the next, there is a significant jump of the image.
The most recent advancement in task-specific lenses is the progressive lens. Although the general-purpose progressive lens rarely works for a full-time computer user (the intermediate zone is too narrow), there are “no-line” types of lenses that work very well. There are several designs for this purpose and most of them have a wide viewing zone to allow you to see your entire workspace clearly. Your eye doctor should be able to make suggestions as to which version might be the most appropriate for your viewing tasks.
Although we are spending more time viewing computers and using our eyes to their maximum ability, it is still possible to be comfortable and productive. Wearing glasses for computer work is just a specific task that requires a specific lens- one that just might help save your eyesight.