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

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Fig. 4.2. Eyeglass adjustments: all of these parts can be adjusted to improve comfort and vision

Finally, the Right Fit

Thank goodness! You’ve run the gauntlet of frames and lenses and made your selections. Now it’s time for the fitting.

First, the optician needs to measure the physical attributes of your eyes. This is to make sure that the lenses will be properly centered over your pupils, and that when the lenses are centered for your eyes, they’ll still fit appropriately in the frame. If you wear a multifocal, the optician needs to measure where the intermediate and near portions of the lens should be placed within the frame. Sometimes the optician will examine your old eyeglasses

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to see how those measurements compare. Good opticians often try to match anything that was unusual about your old glasses to make your transition to the new ones as smooth as possible. The eyeglasses are then sent to a lab so that the lenses can be ground and set into the frame.

When your new glasses are completed, the optician will shape the frame to fit the bridge and temples to your face, for optimal comfort and vision. Keep in mind that there’s almost always an adjustment period for new eyeglasses; this may last from days to weeks. (Most likely, even if your new eyeglasses aren’t a new prescription, you’ll still experience a brief period of adaptation just because of the new frame and fit.) It’s definitely not unusual to experience a few aches in your eyes at first. However, these aches should not be unbearable, and they should diminish over time. Often a quick trip back to the optician to readjust the frame can make all the difference in the world. But if this doesn’t help—and if your vision is clear with the new prescription and the comfort level is bearable—try to stick with the frames for two weeks before returning to your doctor to recheck the prescription. Give your eyes time to adjust.

The need to adapt to new eyeglasses is especially demanding, and often annoying, for those who wear multifocals. One of the easiest and most pleasant ways of adjusting, if you’re getting used to multifocals for the first time, is simply to curl up with a good book. As you read, occasionally look up across the room. This will help your eyes to learn to move smoothly through the lens. Then,

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once you feel comfortable with reading, give yourself and your eyeglasses new challenges. Take a walk around your home. Because moving around with new bifocals—as you may soon find out—is often easier said than done, practicing in a familiar environment makes the adjustment easier. Finally, know that your predicament is only temporary. You really will get used to your new glasses. After you do, you won’t spend your time thinking about “how to use them”; you’ll just live your life and do what you need to do, and you’ll see better than you did before.

When Something Goes Wrong

Okay. Having said all that, we also need to address the fact that sometimes in the world of eyeglasses, as in life, there are glitches. So, you’ve just picked up your new eyeglasses and they’re less than great. Something doesn’t seem right. You had such high hopes: you thought that you would see better, that the world would look and feel better than it had in a long time. And that’s not the case at all. What happened?

The first thing to do is check with your optician and make sure these glasses were made to the doctor’s prescription. Frankly, this is almost always a formality; almost invariably the optician will tell you that the lenses are the correct power and that you’ll need to return to your doctor to have the prescription rechecked.

However, there’s much more to seeing clearly and comfortably than just the power of your new lenses.

When eyeglasses are made for you, the frames and

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lenses need to be measured and fit to tailor the glasses to your face and eyes. Every lens has an optical center, the part of the lens where the prescription is the purest and most precise. The optical center should be located exactly over your pupil, and this can happen only if the frame fits properly and the lenses within that frame are perfectly aligned.

When you have a problem with your new glasses that doesn’t go away over time and the optician tells you that the eyeglasses are made “to the doctor’s prescription,” you need to return to your doctor to have both the eyeglasses and your prescription rechecked. You may understand the problem better if you arm yourself with some working knowledge of “optical jargon.” To that end, we’ve included a brief primer to help you translate some key terms often bandied about the optical shop.

An “Optical Jargon” Primer

“PD”: PD stands for “pupillary distance,” the distance between the centers of your two pupils. This measurement can be taken with a ruler, but it’s most accurately calculated with an instrument called a corneal reflex pupillometer, or CRP. Using the lights, lenses, and prisms on the CRP, your optician can shine a precise point of light at the center of your pupil. First, the optician will place the CRP over your eyes and ask you to look at a light inside. Then, using that light’s reflection off your cornea as a means of alignment, the optician can measure the distance from each eye to the center of the bridge of your nose. Because most people are not perfectly

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symmetrical, it’s not unusual for one eye to be closer to the bridge of the nose than the other. (This becomes important when measuring for progressive addition lenses, where the optical center of each lens must be exactly over each pupil.)

Induced prism: When you look through a lens that is a prism, whatever you see through that lens is displaced. Say, for instance, you’re looking at a ball. The ball may appear to be in one place, but it’s actually located a few inches or feet over. The ball looks perfectly normal—the size and clarity don’t change, in other words—it’s just shifted a little. If problems with your eye muscles hinder your eyes from working together precisely, prisms can make a world of difference. They can realign the images each eye sees so that your brain can interpret the world as one image instead of two.

However, sometimes the problem isn’t your eyes at all but your glasses. If the optical centers of your glasses aren’t properly calibrated over your pupils—when, as opticians say, the PD is not correct—this too can create prism. Oops! When prism isn’t intentional—to clear up an eye muscle problem, for instance—your vision can be distorted or doubled. This is called induced prism. The farther your pupils are from these optical centers, the greater the “shift”; with stronger prescriptions, the prism problem is even more noticeable. For example, say two people have their new glasses made incorrectly, with the optical centers of their lenses made just 5 millimeters wider than the actual distance between the pupils (5 millimeters may not seem like much, but in this precise sci-

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ence, every millimeter counts). The person with the stronger prescription will have the worse case of prism misalignment. (Prism is discussed in more detail in chapter 2.)

Refraction: This is a physics term describing the way light bends as it passes through a lens. (Refraction also describes the procedure your doctor performs when determining your prescription.) When you wear glasses, the lenses bend the light to correct your eyes’ refraction problems and refine your ability to focus. (Refraction is discussed in more detail in chapter 2.)

Reflection: This term denotes the bouncing of light off a surface. Light that reflects off your eyeglasses can create glare.

Diffraction: Another physics term, this refers to how light scatters when it meets a surface (think of sunshine on a lake, for instance). On a bright day, light can scatter at the edge of your eyeglass frames and create glare. Cataracts can create the same problem: by diffracting the light that enters your eye, they can cause glare within the eye and make your vision seem hazy.

Base curve: Think geometry here. Base curve is the curvature of the front surface of a lens. The total power of a lens equals the sum of the front surface curvature and the back surface curvature of the lens. You can vary the lens curvatures for a given prescription but keep the power of the lens constant. For any prescription, the lens can be made with many different base curves. (For more on how an incorrect base curve can cause problems, read on.)

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Pantoscopic tilt: This term describes how the top of your frame angles away from your face. The frame should be fit with a slight pantoscopic tilt to compensate for a downward posturing of the eyes while reading. Too much pantoscopic tilt, however, will move the optical center farther from the pupil and thereby change your prescription!

Retroscopic tilt: This is the opposite of pantoscopic tilt, when the bottom of your frame is angled away from your face, making the top of the frame sit closer than it should. Retroscopic tilt is a bad thing because it distorts vision for both distance and near viewing.

Face form tilt: This term refers to how your frame wraps around your face. Too much face form tilt effectively changes the position of the optical centers of the lenses and results in a distorted view of the world.

Progressive addition lenses: Discussed above, this is the most common type of no-line bifocal, featuring a progressive change in power from the top to the bottom of the lens. The top of the lens, of course, is for your distance vision. The middle of the lens is referred to as the intermediate channel; this provides vision at distances of approximately two to six feet. As you gradually look down into the reading portion of your lens, objects that are closer to you come into focus. These lenses do a great job except when you glance to the side, where the world can appear wavy or even tilted. This is entirely normal, the price of the progressive optics, and simply part of the nature of these lenses. Note: In recent years major strides

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have been made in decreasing the size of this distorted area.

Blended bifocals: They’re not as common as progressive addition lenses, but blended bifocals are also used to create a no-line bifocal. The concept here is basically that of a lined bifocal, except that the line is smoothed out for cosmetic purposes. Someone looking at your eyeglasses can’t see any telltale bifocal line, but you yourself aren’t so lucky. You actually see much more of a line than with regular lined bifocals. In smoothing the line, the blending process seems to widen it, leaving a wide area of distortion in the lens.

Seg height: That’s “seg” as in “segment.” Seg height is where the bifocal segment sits in a lens. For most people who wear lined bifocals, this line of demarcation is at the top margin of the lower eyelid. In progressive addition lenses, the optical center of the distance part of the lens is placed over the pupil, and the amount of seg height is predetermined by the manufacturer of the lens.

Vertex distance: The distance at which your frame holds the lens from the eye is the vertex distance. If you push your eyeglasses in or pull them down on your nose, the prescription will shift away from what it’s supposed to be. Your doctor’s prescription usually assumes that your frame will sit 7 to 10 millimeters from your eye.

High-index lenses: All lens materials have what’s called an index of refraction: how much light is bent, or refracted, when it passes through the lens. A high-index lens bends light much more powerfully than a standard

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lens of the same curvature and thickness—which means that, compared with a standard lens, the high-index lens is thinner (and, to some, more cosmetically appealing).

Polarizing filters: When a light wave bounces off a surface, it becomes polarized: it splits into horizontal and vertical components. The horizontal component of the light wave is what we see as glare. Glasses can be made with a filter that screens out only the horizontal light waves to reduce glare. Such filters are especially helpful for people who spend a lot of time driving or on the water.

Surfacing: This is the term for the manufacturing process that produces a lens blank, a large round lens (approximately 75 millimeters in diameter) with a specific prescription. The lens surfacing machine polishes both the front and back surfaces of the lens to create the desired lens power.

Edging: Edging is the process of grinding a lens blank to be fit into a frame. The lens blank is placed in the edging machine, or “edger,” and aligned according to (1) the lens’s optical center, (2) the person’s PD, and (3) the size and shape of the frame. The edger follows a specific frame pattern to grind, bevel, and polish the lens edges, so that each one fits exactly into the frame.

Troubleshooting Problems with Your New Glasses

Now, what do all of these factors mean to someone whose new eyeglasses don’t work? If your doctor finds that the prescription is correct and that the lenses have been made correctly, then obviously, something else is

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wrong—and chances are, the culprit is one of these features.

Incorrect PD

If the PD is off, for instance, you might experience blurred vision, aches in the front of your head or in your temples, a pulling or drawing sensation between your eyes, dizziness, reading problems, and possibly double vision. The induced prism (see above) does not allow for your eyes to work together, and your eye muscles will probably ache from trying to compensate for the incorrect eyeglasses.

In a progressive addition lens, an incorrect PD adds another problem. When we read, our eyes need to “converge,” or turn in slightly, so that we can focus with both eyes together on the page. When you wear a lined bifocal, you’re measured for a distance PD and a near PD. (The difference between the distance PD and the near PD is called the inset.) The near PD takes the needs of reading into account: it allows for the optical centers to be a little closer together, to match how your eyes converge when you read. With a progressive addition lens, the near PD is preset by the manufacturer and is designed so that each eye individually can follow the progressive path down through the lens into the reading area. If a lens is made incorrectly for the distance PD, this could throw off your ability to read: your eye might not fall within the appropriate area of the lens when you look down to read, and the vision will be blurry. When this is the case, you may be able to read with either eye at a time