Ординатура / Офтальмология / Английские материалы / The Eye Book A Complete Guide to Eye Disorders and Health_Cassel, Billig, Randall_2001

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but not with both eyes together, because the image is not clear for both eyes at the same time.

Problems with Frame Adjustments

Occasionally, even with a correct PD you may experience symptoms similar to those of an incorrect PD. If, for instance, your old eyeglasses were made with an incorrect PD and you adjusted to the induced prism (see above), then you might miss that prism when your eyeglasses are made to match your eyes appropriately. The same is true if, because of ill-fitting frames, the optical centers are placed somewhere other than over your pupils.

In either of these scenarios, your symptoms tend to be more vague than when the PD is made incorrectly. Your eyeglasses “just don’t feel right,” but your vision seems otherwise normal. If the optician hasn’t already done so, your doctor needs to determine whether the frame sits unevenly on your face, and he or she should look at you from four different views.

The first is face-on, to see if the frame is tilting and causing one lens to sit higher than the other. (For example, do you have one ear that’s higher than the other, making the frame tilt?) If you wear a bifocal, frame tilt might cause the bifocal segment line to cross one pupil and create double vision.

The second view is to look from overhead, to see if one lens sits closer to your face than the other. This pushes one of the lenses farther from your eye, which gives that lens a different vertex distance. One eye’s prescription will be less appropriate because the vertex distance will

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be off, and you’ll be uncomfortable seeing with both eyes together. Your vision might be blurry or double.

The third view is your profile for pantoscopic tilt. Too much frame tilt can distort your vision. This extra tilt actually changes your prescription, because the optical centers of the lenses aren’t where they’re supposed to be.

The fourth view is again from overhead, to determine whether your frame has too much face form tilt and is wrapping around your face excessively. This tends to make the world look bowed in or out.

Generally, if the frame fit is different from the fit of your old eyeglasses, it can affect both your vision and your comfort with the new eyeglasses. Always take your old eyeglasses with you when you return to your doctor to have the new glasses checked. This way your doctor can consider how your old eyeglasses fit and try to readjust your new ones to match. Of course, if your old eyeglasses fit poorly but you adjusted to them, you don’t necessarily want to have your new eyeglasses adjusted so that they fit as poorly as the old ones! However, you might have to compromise the fit of your new eyeglasses somewhat to improve your comfort. If your doctor is not comfortable readjusting your frame or doesn’t have the equipment to do so, ask the doctor to write a note for the optician describing specific adjustments that need to be made.

Base-Curve Problems

If the base curve in your new eyeglasses is different from what you’re used to, this can produce symptoms similar to those caused by too much face form tilt: the

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world will look curved or bowed. An incorrect base curve can affect your mobility: your new eyeglasses will make it difficult for you to negotiate curbs or steps while walking. If this is the problem with your new glasses, you may feel that the vision “just doesn’t seem right” without being able to pinpoint your specific symptoms.

Like any eyeglass problem, base-curve problems are more common in higher prescriptions and are usually not experienced when the prescription is mild. If you can’t adjust to the new eyeglasses, the optician will have to remake your lenses to match the base curve of your old pair.

Problems of Having a Different Prescription for Each Eye

When everything has been made correctly with your new eyeglasses but you still have complaints, the problem might relate to the prescription itself. That is, a prescription may simply be difficult to get used to with your eyes together, even if it’s correct for either eye by itself. If the prescriptions for your two eyes are different, then you have a condition called anisometropia. (The symptoms of anisometropia are that one eye’s image seems farther away or a different size than the other eye’s image. Someone with this condition may also see double when viewing with both eyes together.)

When your eyes are different, the placement of the lenses over your pupils becomes critical for clear and comfortable vision. If, for example, your eyes sit evenly above the optical centers of the lenses and one lens is

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stronger, then that lens will have more induced prism at that eye’s pupil than the other lens. This uneven induced prism will make your vision seem distorted, create dizziness and discomfort, and create a doubling of the world so that you see two images, one on top of the other.

A more common problem with each eye having a different prescription is image jump. In a lined bifocal, as you move your eyes down to the line, the image will appear to “jump.” When both eyes have the same prescription, the jump isn’t really noticeable once you adapt to your eyeglasses. But when your eyes have different prescriptions, the jump is often greater in one eye than in the other, and thus more conspicuous. If your visual system isn’t able to recover from this difference in jump, you may experience double vision when you read. Otherwise, you might describe your problem by saying that “the print swims, moves, or dances” on the page, or that you don’t feel like your eyes are “working together for reading.”

For these reasons, if you have a different prescription in each eye, you may not be able to wear a bifocal. If having two pairs of eyeglasses—one for distance and one for reading—is too inconvenient, a progressive addition lens might alleviate the symptoms of image jump that you had with a lined bifocal. Occasionally, symptoms of image jump can be relieved with a special type of prism called slab-off prism. This is incorporated into one lens of a lined bifocal to counteract the difference in image jump created by different prescriptions. Contact lenses

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are another solution: they greatly reduce the effects of different prescriptions and offer an excellent alternative for your vision (see chapter 5).

Problems with Materials

Eyeglasses that have been measured and made correctly with an appropriate prescription still may not provide you with the vision that you were expecting. Keep in mind that there are many different materials and lens qualities available on the market today. Not all plastic lenses are created equal, and some lenses are clearer than others.

Plastic lenses are made of a material known as -39, which is exceptional for transmitting light without distortion and provides an excellent optical-quality image. The Food and Drug Administration (FDA) requires that a lens manufacturer needs to use at least 50 percent of this material in a lens in order for that lens to be considered a “ -39 lens.” Different manufacturers use different proportions of the material in their lenses. The finest plastic lenses are 100 percent -39; however, they also tend to be more expensive. If you’re looking for the least expensive prices, you may inadvertently be buying an inferior lens. The problem is that you never really know what you’re getting. In general, if the optician orders a stock lens from a major lens manufacturer rather than manufacturing lenses on site, there is less likely to be a problem. (Keep in mind that manufacturing lenses and edging lenses to be fit into a frame are two different processes.)

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Lenses are also made of many materials other than - 39. Newer plastic lens materials that look much thinner and feel much lighter can be used to make your eyeglasses look and feel better. Many advertisements for these lenses have made eyeglass wearers aware of their availability. As always, some of these materials work better than others. The first such material to become available was polycarbonate. Originally these lenses were designed to be used in safety eyewear that required a high-impact-resistant lens. Because these lenses were so much lighter in weight than the high-index glass lenses in use at the time, they gained in popularity very quickly. Early on, however, it was discovered that these lenses didn’t provide the same quality of image as did standard-39 lenses; to make matters worse, this shortcoming was much more noticeable when the prescription was stronger. So people who needed stronger prescriptions were more likely to spend more money for these lenses, only to be disappointed by their vision with the new eyeglasses. Newer polycarbonate materials apparently do not have this problem, but as with -39, not all optical laboratories use the same quality lenses. Over the past few years high-index lenses that are made of plastic materials other than polycarbonate have become available. While these materials are not totally free of problems, in general they provide better vision than polycarbonate.

Lens coatings are sometimes the source of complaints with a new pair of eyeglasses. The most common lens coating used today is an “antireflective” coating, which reduces the amount of light reflecting off the lens surface

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and thus decreases glare. When properly applied, an antireflective coating can improve vision. But a poor-qual- ity antireflective coating will leave noticeable ripples in the lenses and thus distort vision.

Other Things That Can Go Wrong

Finally, some things that go wrong with eyeglasses revolve around the wearer. Here are three things you can do to help avoid these problems.

First, be careful when you choose a new frame. The optician should be skilled at helping you to find the best fit for your face and prescription, but sometimes—how can we put this diplomatically?—a sale becomes more important than offering the best advice. If your frame is too large, it may create a distorted view of the world and affect your ability to navigate while wearing your eyeglasses. If the frame extends past the side of your face on either side, it can create glare from light reflecting off the back surface of the lenses.

Second, be careful to describe your vision needs to your doctor. This is of utmost importance. Your doctor can prescribe eyeglasses based on the average person’s needs but can tailor your prescription to fit how you use your eyes during the day. The biggest problem with bifocal prescriptions, in particular, is determining a reading correction that allows you to read at the distance that you require. Every reading lens power focuses light at a given distance, and as the reading power increases, the reading distance gets shorter. We don’t all necessarily have the same reading distance. Our reading distances ei-

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ther come from habit or are specific to a job or hobby. For example, if you sit all day with a computer monitor twenty-four inches away from you, and your doctor assumes that you like to read at sixteen inches, then your eyeglasses won’t be very helpful for the computer unless you move much closer to it.

The final thing you need to look at requires some introspection. We’ve all bought things—clothes, jewelry, and the like—that have gone over like a lead balloon with our friends and family. Many of us respond to this decided lack of enthusiasm by declining to wear that particular thing again. If for some reason you come to hate your new frames, then subconsciously you might not feel as if you see well. You may secretly be hoping that the eyeglasses can be returned. Try to be honest with yourself, the optician, and the doctor—because all of you will spend a lot of time trying to find out the source of your symptoms.

As a last word of advice, always have your doctor write a note for you to take back to the optician describing exactly what’s wrong with your new eyeglasses. This helps you to communicate the problem to the optician effectively; it also adds more credence to the problem at hand. Without a note, the optician might just dismiss the problem and tell you that you don’t really understand enough about eyeglasses to know what you’re talking about.

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Some Questions You May Have about Eyeglasses

Does it matter where I get my new glasses?

In a perfect world, every optical shop, optician, optometrist, and ophthalmologist would treat you the same, radiating infinite concern that your new eyeglasses provide ideal vision, comfort, and looks—all, of course, at a reasonable price. Unfortunately, as in any profession, there are some among the spectacle purveyors who can be considered unscrupulous. Some chain stores are the epitome of professionalism; others illustrate in the worst possible way the principle of “caveat emptor” (let the buyer beware). The same can be said for some professional opticians. So the first thing to know is that the individuals you’ll be dealing with—their knowledge and their ethics—matter much more than the location. In this regard, word of mouth can be very helpful. Most likely, human nature being what it is, someone who has gotten lousy service will be more than happy to tell you all about it, and someone who has received excellent care from a particular eye doctor or optician will enjoy spreading that word as well.

There are several advantages to buying your eyeglasses at your doctor’s office—if, of course, the price is fair. For one thing, you have the advantage of having your doctor available to recheck your prescription if something doesn’t seem right. Also, if the new prescription needs to be changed, your doctor will probably not charge you for the second set of lenses. While some doctors tell their pa-

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tients “old wives’ tales” about “getting used to your new glasses” just to avoid remaking them, most doctors feel responsible for any lab work produced by their office. For them, this craftsmanship reflects the values of the office as a whole, and they’ll do anything to make sure their patients are happy with their new eyeglasses.

Eyeglasses bought elsewhere might be more convenient for you if the optician’s shop is located where you can easily stop by for frame adjustments. However, if there’s a problem, you’ll find yourself traveling between the optician’s and the doctor’s offices, probably several times, until the problem is resolved. The “one-hour” opticians can be a lifesaver if you really have an emergency need for new eyeglasses. However, some of these establishments manufacture their own lenses, and the quality of materials may not provide you with the best vision possible.

Ultimately, you need to shop where you’re comfortable or have had good references. If you know and trust your doctor, and the doctor’s eyeglass prices are pretty much in line with prices elsewhere, it’s probably worth getting your glasses made there for the service and quality of the eyeglasses. Remember, most people wear the same pair of glasses every day, usually for two years or more. No other piece of your wardrobe receives as much wear. It’s probably worth a little more of your time, and sometimes your money, to make sure that you have the best in vision, comfort, and looks with your eyeglasses. (In other words, it takes a reasonable length of time to make your glasses right the first time, and the people