Eye Care Blog

When you get glasses for the first time, or when there is a change in your prescription, it will upset your visual world. Certainly you can expect clearer sight and more comfortable vision, but along with that you will notice some strange side effects. Objects may appear larger or smaller, closer or farther, and familiar shapes may be deformed. Don’t hit the panic button. These effects are only temporary and within a few days you should be completely unaware of them. But why the problem in the first place? There are several reasons.

Because of technical limitations, only the very center of the lens has the exact prescription your eye needs. When you look away from the center (and you do every time you move your eyes), you’re actually looking through a slightly different prescription. This difference increases towards the edge of the lens and causes shapes and sizes to be distorted. The thicker the lens, the greater the distortion; the larger the lens, the more edge distortion. (The eye is increasingly farther from the lens margin.) In low power prescriptions, the warping of objects and space is manageable and you can adjust quickly. With high powers, the adjustment can be very difficult, and large size glasses should be avoided.

Another factor influencing your sight with glasses is the curvature of the lens surfaces. It is possible to produce lenses having the same prescription power but with a variety of curvatures. A lens can be made with one flat surface, two modest curves or nearly bulging. So what? The curvatures affect the image size and shape seen through that lens. For example, a concave lens for a nearsighted person will produce a larger image if made with a deep curve. If the prescription in your two eyes is quite different, it is no easy matter to select matching curvatures.

Has it ever happened to you that two supposedly identical pairs of glasses don’t feel the same? It could be that they were made with lenses of dissimilar curvatures.

If you are nearsighted and put glasses on for the first time, the world will look clearer but smaller. Since the brain uses size as one judgment of distances (small objects are assumed to be farther away), you will tend to think you are more remote from things than you really are. You may spill a cup of coffee when you reach for it because it’s closer than you visualize.

If you are farsighted, the opposite will happen. Objects will appear clearer but larger. So, you will judge things to be closer and when you reach for a doorknob you may come up short. Over the span of a few days, the brain will recalculate object/ distance relationships and you will regain your familiar distance judgement.

These annoyances are minor compared to the first-time correction for astigmatism. Objects will be tilted and curved; your entire perspective will be in shambles. (You’ll appreciate what fictional Alice experienced.) Be courageous—as confusing as things may be, they will slowly reassemble into a “normal” looking world within a few days.

Scientific and technical advancements in lens materials now makes it possible to produce thin, lightweight lenses out of high- index resins. The higher the index of refraction of a lens material, the more prescription power can be squeezed into a given area. The result is a dramatic, cosmetic improvement in the way the finished glasses look. However, because the prescription power is intensified, moving the eyes off the lens center will cause a quicker change in effective strength. It will take a little longer to adapt to glasses with these materials, but it’s well worth it.

A seemingly minor matter such as the distance between the lens and your eye will effect the power and image size. If you are farsighted and the glasses slip down your nose, the effective power and image size are increased. If you are nearsighted and wear your glasses at the end of your nose, the effective power is reduced. The exact amount will vary with the length of your nose and the prescription.

All the annoying symptoms are aggravated proportionally with high power prescriptions. The very nearsighted person also has to contend with unsightly, thick, light-reflecting edges on the lenses. While these can be reduced with the high-index lenses, a further improvement can be made by “rolling” the edges and tinting the lens. A better way is to choose a smaller frame because the edge thickness increases sharply with increased lens size.

A very farsighted person need not be concerned with a thick edge, but the eyes exhibit an enlarged “bug-eyed” look from the bulky center thickness. This can largely be overcome with special high-index materials which are designed with unique curvatures to flatten the center bulge. Of course, the adaptation time will be longer.

Some people are bothered by reflections from the surfaces of the lenses, especially at night. You should have little trouble learning to ignore them, but if desired, the reflections can be mostly eliminated with a multilayered, anti-reflective coating on the surfaces.

To get the best vision, the centers of the lenses must be positioned directly in front of your pupils. Rarely does the frame size exactly match this position. Therefore, the lenses must be offset to compensate for the actual eye placement. If this is not done, it can disturb your eye movements and focusing.

For all these reasons, it should be apparent that the making of glasses must not be a haphazard procedure. The measuring, fitting and aligning of the glasses has to be done accurately and knowledgeably. For your own good, the optometrist should oversee the entire process.

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