Our eyes work as "live cameras" for the brain,
gathering up and processing images far better than any high-tech device.
Everybody understand the value of vision. Eyesight accounts for about 75 percent of our total perception, it is the most important way we experience life. But as with our other senses: smelling, hearing, tasting and touching, we sometimes take seeing for granted, not realizing that proper eye care, including regular professional examinations, is the key to long term good vision. Let's have a closer look at our eyes and how they work, common vision problems, and wearing corrective lenses in general.
Housed in a sphere smaller than a ping-pong ball, the eye is a complete optical system that transforms light into the images we see. Our eyes work as "live cameras" for the brain, gathering up and processing images far better than any high-tech device.
Because the eye is so complex, defects are bound to occur. It is estimated that as many as 90 percent of us have at least slightly imperfect eyesight. About 60 percent of us need corrective lenses part, if not all of the time. Only a few enjoy perfect vision.
The human eye is an opaque ball with a transparent bulge providing a clear "window" at its front side.
The eye is about an inch in diameter and weighs just a quarter of an ounce.
Within the eye is an intricate arrangement of tissues, fluids, nerves and cells.
The clear, dome-shaped "front window" of the eye.
The cornea is a lens that bends (refracts) light rays as they pass through and accounts for about 70 percent of the eye's total focusing power.
The "white of the eye."
Along with the cornea, it forms a tough protective coating.
The sclera continues back over the optic nerve to join with the outer covering of the brain.
The doughnut-shaped ring of pigmented tissue that determines an eye's color.
The iris opens and closes to control the amount of entering light.
The pupil is the hole in the center of the iris that appears black.
In dim light, the iris enlarges the pupil, increasing the amount of light entering the eye and improving vision.
In bright light, the iris reduces the pupil's size to decrease entering light and avoid eye damage.
The pupil looks black because it is very dark inside, almost no light is reflected back out.
The transparent tissue that acts like a magnifying glass behind the pupil.
The crystalline lens flexes when we want to look at something close-up, providing about 30 percent of the eye’s total focusing power.
The growth and hardening of the lens causes it to lose its flexibility over time, which is why people over 45 usually need bifocal contacts or glasses, or reading glasses.
The membrane lining the inside of the eyelids and the sclera (white part of the eye).
It firmly attaches the eyeball to the eyelid and eye socket, but it is flexible enough to permit us to move our eyes up and down and side to side.
The conjunctiva protects the eye from foreign particles and some viruses and bacteria.
The watery fluid filling the space behind the cornea and forward of the crystalline lens.
Its main function is to provide nutrients to the front portion of the eyeball.
The layer of light-sensitive nerve cells lining much of the inside of the eyeball.
The retina contains receptor cells called rods and cones that convert light into electrochemical impulses sent to the brain.
Rods aid vision in dim light, while cones help with color perception.
The area in the middle of the retina responsible for distinguishing fine details and colors.
At its center is the fovea, a tiny pit containing the highest concentration of cones and providing the ultimate focal point for the optical system.
The "conduit" of nerve fibers that carries impulses from the retina to the brain.
The optic nerve is derived from optic stalks during the seventh week of development and is composed of retinal ganglion cell axons and glial cells.
In humans, the optic nerve extends from the optic disc to the optic chiasm and continues as the optic tract to the lateral geniculate nucleus, pretectal nuclei, and superior colliculus.
The clear, jelly-like substance filling the otherwise empty space behind the crystalline lens.
It serves primarily to keep the retina pressed against the inside wall of the eyeball.
The middle layer of the eyeball’s casing, positioned between the sclera and retina.
It supplies most of the retina’s nourishment and has one of the highest blood flows in the body.
The extension of the choroid, connecting with the iris.
It produces the aqueous humor and contains the muscle system that controls the flexing of the crystalline lens.
The ciliary body is connected to the lens by fine fibers called zonules.
The process of seeing is often compared to taking pictures with a camera.
In both cases, light rays reflected from objects within view are gathered up, focused and converted into images.
Primary mechanical parts of the eye and the camera are particularly comparable.
The eye's cornea and crystalline lens combine to provide focusing power just as a camera's lens does.
The iris operates as a diaphragm, reacting to brightness or dimness of incoming light with "settings" like those made to change a camera's shutter speed.
The crystalline lens flexes to sharpen focus in a way similar to adjusting a camera lens backward and forward.Comparisons do not stop with optics.
The retina acts like a camera's film, converting light into a form that can be made into pictures. "Film processing" is accomplished in the brain at a pace much faster than any high-tech photo lab. Ultimately, the eye and the camera have a basic fact in common: a good finished image requires precision-made parts that are well aligned. And both depend on good refraction.
Focusing begins as soon as waves of light arrive at the eye's front surface, at the cornea.
The cornea functions as a lens, and refraction occurs at both its front and back surfaces.
This critical gathering and bending of light also serves to converge the scattered waves into a single beam, aiming it to travel on course. About 70 percent of refraction occurs here.
After passing through the watery aqueous humor behind the cornea, light continues into the eye through the pupil.
The size of the pupil is determined by the surround-ing iris, the ring of colored fibrous tissue.
In bright light, the pupil reflexively constricts, and in dim light it expands, to regulate the amount of light reaching the retina.
Finishing touches to refraction occur in the crystalline lens, a capsule of trans-parent cells suspended just behind the pupil.
Often called just the lens, it acts like an adjustable magnifying glass, reflex-ively thickening to increase its refractive power when we want to focus close-up.
This function is called accommodation, in which muscles in the ciliary body contract to relax the lens into a more convex shape.
Most of the 30 percent of refraction not provided by the cornea is accommodated by the lens.
Light waves get a final bit of refraction as they leave the crystalline lens and enter the vitreous humor.
On their straight-line journey through this clear, jelly-like substance, the waves converge at a single point before diverging to project an inverted (upside down) image on the retina. (The brain knows to perceive the image right side up.)
Once light waves reach the retina, they are converted into electrochemical impulses by light-sensitive nerve cells.
The retina contains several types of cells—also referred to as photoreceptors or photosensors, that perform this conversion. Most important among them are cones (named for their shape), which work to distinguish colors and fine detail. Highly concentrated in the fovea (the bull's eye niche in the center of the macula), they work best in bright light. Also named for their shape are rods, which are important for peripheral vision and which work well in dim light.
Transmission and interpretation.
The steady stream of electrochemical impulses coming from the retina is conducted to the brain via the optic nerve, which is really a "cable" comprised of bundled nerve fibers.
The brain reads or interprets these electrochemical impulses as images.
The term 20/20 vision refers to how well a person sees at a distance. Specifically, it means one can see clearly at 20 feet what should normally be seen at 20 feet. Someone with 20/100 vision must be as close as 20 feet to see what a person with normal vision sees at 100 feet. Someone with 20/10 vision sees better than a person with normal vision—what’s clear to that person at 20 feet will be clear to someone with normal vision at 10 feet. Many people with less than normal distance vision see 20/20 with the aid of contacts or glasses.
People with 20/20 vision do not necessarily have perfect vision. Perfect vision also includes elements like near vision, peripheral vision, eye muscle coordination, depth perception and color distinction.
Considering the intricacies involved in the anatomy and function of the eye, all contained in a one-inch ball, it is easy to imagine how problems can occur. As with a camera or any other optical system, the tiniest deviation, a fraction of a millimeter here, a fraction of a degree there—will throw things off enough to spoil the picture.
Common vision problems involving imprecise formation of the eye's optical parts are called refractive errors. There are four basic types: myopia (nearsightedness), hyperopia (farsightedness), astigmatism (uneven cornea) and presbyopia (aging eye).
Myopia :: Near sightedness.
Myopic or nearsighted people can see near objects clearly, but distant objects are out of focus.
The main cause is an eyeball that is longer than normal from front to back. The extra length means light from distant objects is focused short of the retina. Light continues on to reach the retina, but the projected image is blurred. Myopia also can occur when the curvature of the cornea is overly steep (or "over bulged"). The resulting extra refraction has the same result as the elongated eyeball: focus falls short of the retina. Some people have both errors.
Hyperopia :: farsigbtedness.
Hyperopia occurs when the eyeball is too short. The reduced length means the point of focus lies beyond the back wall of the eye, and light rays are not yet in focus when they arrive at the retina. The result is a blurred image. Improper corneal curvature can also cause hyperopia. If the cornea is too flat, not enough refraction of light rays from near objects takes place to bring them into focus on the retina.
Normally, all young children have a small degree of hyperopia that decreases as they progress through their teens. Despite their hyperopia, most youngsters can see well at all distances because the accommodation provided by the crystalline lens is enough to counteract minor refractive errors.
As we get older, the crystalline lens in each of our eyes loses its suppleness and becomes less flexible. When this occurs, our ability to focus close-up diminishes, and our eyes are said to be presbyopic.
This condition usually becomes noticeable between the ages of 40 and 50, and people with otherwise normal eyesight find it increasingly difficult to read or do close up activities without bifocal contacts or reading glasses. Many develop the need for bifocal or multi-focal lenses to see clearly at all distances.
Astigmatism is an overall inability of the eye to focus clearly at any distance because of uneven curvatures of the cornea. Essentially, the cornea is oval, having a surface shaped more like a football or the back of a spoon rather than being rounded like a basketball. Very frequently associated with myopia, astigmatism can cause light rays to focus at more than one point on the retina, and objects at all distances appear distorted much the same way as some fun house mirrors make images too tall, wide, thin, or short.
Virtually all corneas have at least a tiny degree of astigmatism. For many, the resulting distortion is not discernible. But as the curvature of the cornea becomes more uneven, image distortion increases. Eyeglasses, hard contact lenses or toric soft contact lenses are used to correct astigmatism. Astigmatism is often inherited.
Historical accounts of the origins and development of contact lenses are full of fascinating anecdotes and debatable claims. Leonardo da Vinci is often cited as being the first to illustrate the concept of contacts in 1508.
In 1960, experiments to make contacts out of water-absorbing (hydrophilic) plastic began, and the first soft lens made of such material became available commercially in the U.S. in 1971.
The first RGP lenses were introduced commercially in 1979. Also called oxygen permeable lenses, RGP contacts are made of a variety of silicone-acrylate combinations. They normally cover about two-thirds of the cornea.
After a gradual breaking-in period, contacts were to be worn each day for "as many hours as you can stand it." Now we have two basic classes according to prescribed wearing periods: Daily Wear Contacts and Extended Wear Contacts.
New materials for extended wear lenses permit more oxygen to reach the cornea, which is essential to eye health. In addition, improved designs allow the lenses to move more freely on the eye so debris does not get trapped under them.
Contact lenses have provided millions of people with an excellent alternative to eyeglasses, and technological advances are making them better and safer all the time.
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Dr. Clarkson founded Discount Contact Lenses in 1995 with a vision to provide a high-quality contact lens replacement service focusing on the then-emerging internet. Dr Clarkson has led Discount Contact Lenses through 12 straight years of profitable growth, allowing the company to develop from a small operation in the back of an optometric office to one of the largest contact lens retailers in the USA. Dr. Clarkson is also President of Eyestyles, a Columbus Ohio optical retailer which he founded.
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