How does our vision change over time?
How our vision changes over time as we age
When we're children, most of us see well enough on our own without needing glasses. But, by the time we reach our forties, that's no longer the case and many of us need glasses or contact lenses to help us see
better. Even then, it can be frustrating. Every few years our prescriptions change and we all wonder when our eyes will finally settle down. Well, guess what? They never do!
From a vision standpoint, our eyes are in a constant state of flux our entire lives. Even after we're fully grown, our eyes keep right on changing. Some of these fluctuations cause a shift in our prescriptions. Some of them cause a change in our vision, due to age-related eye disease.
What changes our eye vision prescriptions?
To answer this question, let's take a minute to review how we actually see. It may help to picture the eye as a camera. The cornea, the front
surface of the eye, acts as a converging lens: it takes rays of light and bends and focuses them.
Next, the light is focused more finely still by the lens, which sits just behind the iris (the coloured part of the eye). In a perfect eye, all of this focused light is then beamed back to the retina-what you might call the eye's "film."
The elegant, complicated retina processes all the information it receives, breaking it down into countless elements, and then transmitting these elements in the form of electrical signals to the brain.
But, as we get older, our eyes become less than perfect. Both the cornea and the lens change shape over time. Accordingly and inevitably, our prescription changes, too.
How the cornea changes over time
When we're born, our upper eyelids tend to press on our and tight and corneas. It has been suggested that this snug fit eases over time, and as it does, the cornea starts to reshape itself. What results is a mini domino effect. The loosening of the eyelids causes the corneas to alter their contour, changing the way our eyes focus light and, consequently, changing the prescription we need to correct this focus.
Often this also causes us to have an increase in astigmatism (a focusing problem) as we reach our forties and fifties.
How the eye lens changes over time
Another issue is that the lens within the eye also changes throughout our lives, starting even before we're born. As it grows, layer after layer of cells - like the rings of a tree or the skins of an onion – add themselves to the front surface of the lens.
This growth pattern too has an impact on our prescription. Older layers of cells within the centre of the lens become more compacted, making the lens nucleus more dense and the lens in general less flexible. Eventually these changes affect our ability to focus on near objects. An increase in lens density is also the most common cause of cataracts.
What are refractive errors?
When you need to wear corrective lenses in order to achieve clear vision, you have what's called refractive error. Refraction is a physics term that refers to how light is bent as it passes through lens. When your eye doesn't do a good job of bending and focusing light onto your retina and,
thus allowing you to see, then there's an error in the eye's refraction ability. Refractive errors for the eye are myopia, hyperopia, astigmatism, and presbyopia.
What is myopia?
Myopia means nearsightedness. A nearsighted eye can focus better on close objects than on distant objects. But this is a relative term, meaning that the degrees of nearsightedness vary considerably. Without corrective lenses, someone who is extremely nearsighted really only sees clearly at too close a distance to be functional, while people who are only mildly near-sighted can generally see well enough to perform most of their daily tasks.
The myopic eye is actually an eye that has too much focusing power. There are three possible reasons for this excess power: either the cornea has too much curvature, or the eye is too long from front to back, or the lens within the eye is focusing excessively.
In all of these instances, light is focused in front of the retina. In other words, it undershoots its mark. Your prescription is designed to have the opposite effect, So that your corrective lens offsets the eye's high power and allows the light to focus directly on the retina.
Eyeglass prescriptions written for someone who's nearsighted a have a minus sign in front of the lens power. This indicates that the lens of
the eyeglasses is effectively taking away the excess power of the eye. A near-sighted eye is not weak; instead, it's too strong.
What is hyperopia?
Hyperopia is the technical term for farsightedness. As you might expect, a hyperopic eye is the opposite of a myopic eye in that it doesn't have
enough power to focus light precisely on the retina. Either the cornea or the lens doesn't have enough curvature, or the eye is too short for light to be focused appropriately. The term accommodation refers to the way the eye muscle and lens work together to focus on something nearby, a book, for example, or a makeup mirror.
Farsighted people use this same accommodative system to compensate for their lack of focusing power, in order to see at a distance. When farsighted people try to read, however, they must focus for their hyperopia as well as for the reading distance - an effort that requires significantly more eye muscle power.
For farsighted people, then, vision is relatively clearer at a long distance than up close, because it takes less muscular effort. For someone who is extremely farsighted, the world really is blurry at all distances. The hyperopic prescription is the opposite of the myopic prescription: there's a plus sign before the lens power, indicating that the eyeglass or contact lens is adding more power to the eye.
What is astigmatism?
Astigmatism may be one of the most misunderstood and misused terms in our optometry. Astigmatism is a refractive problem in which an eye doesn't focus light evenly. A short lesson in physics may help illustrate this. Imagine that you have a tiny light bulb, the size of a pinpoint. This light sends rays out equally in all directions, forming a sphere of light.
If this light is focused through a lens that is also spherical-in other words, if it's curved equally in all directions like a basketball then this light, when focused onto a flat surface such as a wall, forms circle, just like the round spot of light you create when you shine a flashlight at a wall.
A lens that has astigmatism isn't curved evenly in all directions. Instead of a basketball, it's like an American football. In an eye that has astigmatism, usually the corneal curvature is greatest in one direction and least in the opposite direction. A football has less curvature across the ball than around it.
Both directions across the ball and around the ball have some curvature, but it's not evenly distributed, as it is on the basketball. If you focus a pinpoint of light through a football-shaped lens, a the light gets focused more around the lens than across the lens, and it exits as a stretched-out circle or oval.
Because the eye's variations in curvature are much more delicate, you really can't see this football shape. But this subtle difference in curvature tends to blur images that we see, so that the world
looks stretched out, just like that contorted circle of light.
Aging and near-sightedness
Here is the typical lifetime course of myopia: As our eyes grow, Over the first twenty-five years or so, near-sighted people tend to become more near-sighted. Also, because so much of these early years is spent in education, the many hours a day spent reading and focusing up close probably add to the problem.
All of the accommodation necessary for this focusing creates a situation in which the ciliary muscles are constantly contracted and the lens is constantly focused at near; eventually the eye can adapt to that contracted state-in other words, by getting used to focusing mostly at a reading distance and become still more myopic.
Between ages twenty-five and thirty-five, the near-sighted prescription usually doesn't change much. However, as you might expect, those of us whose work involves extensive reading, writing, or other up-close focusing may still need increasingly stronger prescriptions during this time.
After about age thirty-five, the eye changes that cause accommodation problems begin to affect our degree of near-sightedness as well. As the up-close focusing system starts to fail, myopia that was brought on by excessive accommodation actually starts to get better; often this trend continues into our late fifties and early sixties. However, because this decrease in near-sightedness is a sign of diminishing accommodation, this
seemingly happy turn of events may simply turn out to be the first step on the road to needing bifocals. Thus, for many people in their sixties there comes, after years of enjoying milder prescriptions, a reality check of sorts: becoming more near-sighted again.
The most common type of cataract, nuclear sclerosis, is a consequence of years of growth and subsequent hardening of the lens. In people with myopia this hardening of the lens at first may tend to increase its focusing power, and once again we start getting more near-sighted.
Aging and farsightedness
With hyperopia, again, there's a typical lifetime cycle of progression. Up to about age twenty-five, farsightedness sometimes gets better on its known, as our eyes grow and develop.
If, for instance, the problem is that the eye is too short for light to be focused appropriately, this might resolve itself when the eye simply gets bigger. Often there's no need to correct this problem in someone younger than twenty-five, because the accommodative system has a tremendous ability to compensate for this kind of anatomical shortcoming.
But, the eye changes that cause us to need bifocals can manifest themselves much earlier in someone who's farsighted, and they can begin to affect our ability to read and focus up close as early as our twenties.
As we get into our late thirties and early forties, when we begin holding the newspaper at arm's length, we also begin to lose the ability to accommodate for our hyperopia. We begin having trouble with bur distance vision and may require corrective lenses to help us to see better.
Glasses prescriptions for distance vision often get stronger in our late fifties and early sixties. For farsighted people the dawn of cataract formation may cause the opposite effect of the change that occurs with near-sighted people: a shift in the power of the lens within the eye causes an increase in the eye's focusing power, which results in a decrease in hyperopia.
Aging and astigmatism
Astigmatism doesn't change as much with age as myopia or hyperopia do. After the eye stops growing, astigmatism levels off; decades may go by before you need a new a prescription.
Then along comes presbyopia and this plus changes in lid tension across the cornea can cause changes in astigmatism that may be for better or worse, depending on your particular case. Early cataract formation can also cause changes, as the cataract subtly changes the shape of the lens within the eye.
Aging and presbyopia
After its onset, presbyopia produces a classic pattern of change over a
the next fifteen to twenty years. Initially your glasses prescription is doing part of the focusing for you. As your accommodative system changes, the glasses prescription increases, until eventually the prescription does all of the focusing for you.
Usually the first symptom of presbyopia is that we can't seem to
bring an object as close to our eyes, and keep it in focus, as we used to. Also, it takes longer to bring a near object into focus and then to shift our focus to look at something across the street or even across the room. The cause of this delay ips the ever-thickening, increasingly less flexible lens, which doesn't do its job as fluidly and effortlessly as it once did.
As presbyopia progresses, the point at which we can focus moves farther and farther away, until eventually anything inside of twenty feet looks blurry. While your presbyopia progresses to the point where your reading addition to your distance prescription does 100 percent of the focusing for you, your near range of focus tends to diminish.
What's happening is that as you need more lens power in your glasses to compensate for your presbyopia, your ciliary muscles and lens are less flexible for range of vision, and your near working distance keeps inching closer, to where the spectacle lens focuses.
Those of us who spend a lot of time viewing objects at intermediate distances of twenty to forty inches- the computer monitor, for example, often need additional lens prescriptions. For many people the best solution is either progressive addition, "no-line" bifocal that allows for multiple working distances, or a trifocal that adds another lens to your spectacles, designed particularly for intermediate-distance viewing.
Early cataract formation, or nuclear sclerosis, also affects presbyopia. Because this causes a shift in vision that can make people either more near-sighted or less farsighted, some of us suddenly find ourselves able to read without our bifocals (a delightful phenomenon often referred to as second sight). Unfortunately, this is a short-term improvement; as the cataract progresses, vision at all levels tends to get worse.