How do lenses correct nearsightedness and farsightedness

Figure 2. Correction of nearsightedness requires a diverging lens that compensates for the overconvergence by the eye. The diverging lens produces an image closer to the eye than the object, so that the nearsighted person can see it clearly. What power of spectacle lens is needed to correct the vision of a nearsighted person whose far point is Assume the spectacle corrective lens is held 1. You want this nearsighted person to be able to see very distant objects clearly. That means the spectacle lens must produce an image An image The image distance is negative, because it is on the same side of the spectacle as the object.

The negative power indicates a diverging or concave lens, as expected. The spectacle produces a case 3 image closer to the eye, where the person can see it. If you examine eyeglasses for nearsighted people, you will find the lenses are thinnest in the center. Additionally, if you examine a prescription for eyeglasses for nearsighted people, you will find that the prescribed power is negative and given in units of diopters. Since the farsighted eye under converges light rays, the correction for farsightedness is to place a converging spectacle lens in front of the eye.

This increases the power of an eye that is too weak. Another way of thinking about this is that a converging spectacle lens produces a case 2 image, which is farther from the eye than the object see Figure 3.

Then the image produced by a spectacle lens must be at this distance or farther for the farsighted person to be able to see it clearly. Figure 3. Correction of farsightedness uses a converging lens that compensates for the under convergence by the eye. The converging lens produces an image farther from the eye than the object, so that the farsighted person can see it clearly.

What power of spectacle lens is needed to allow a farsighted person, whose near point is 1. When an object is held An image 1. The object is The positive power indicates a converging convex lens, as expected. The convex spectacle produces a case 2 image farther from the eye, where the person can see it. If you examine eyeglasses of farsighted people, you will find the lenses to be thickest in the center. In addition, a prescription of eyeglasses for farsighted people has a prescribed power that is positive.

Figure 4. This chart can detect astigmatism, unevenness in the focus of the eye. Check each of your eyes separately by looking at the center cross without spectacles if you wear them. If lines along some axes appear darker or clearer than others, you have an astigmatism.

Another common vision defect is astigmatism , an unevenness or asymmetry in the focus of the eye. For example, rays passing through a vertical region of the eye may focus closer than rays passing through a horizontal region, resulting in the image appearing elongated.

This is mostly due to irregularities in the shape of the cornea but can also be due to lens irregularities or unevenness in the retina. Because of these irregularities, different parts of the lens system produce images at different locations.

The eye-brain system can compensate for some of these irregularities, but they generally manifest themselves as less distinct vision or sharper images along certain axes. Figure 4 shows a chart used to detect astigmatism. Astigmatism can be at least partially corrected with a spectacle having the opposite irregularity of the eye.

If an eyeglass prescription has a cylindrical correction, it is there to correct astigmatism. The normal corrections for short- or farsightedness are spherical corrections, uniform along all axes. Contact lenses have advantages over glasses beyond their cosmetic aspects.

One problem with glasses is that as the eye moves, it is not at a fixed distance from the spectacle lens. Contacts rest on and move with the eye, eliminating this problem.

In the center of the iris is the pupil , through which light enters your eye. Just behind the pupil is the lens, whose job it is to focus the light entering your eye. That light falls onto the retina at the back of your eye, which is a complex layer of cells that react to light. The optic nerve carries those signals to your brain, where they are decoded into an image. First, your prescription will say OD and OS.

These are abreviations for the Latin oculus dextrus , the right eye, and oculus sinister , the left eye. Sometimes, your prescription will also contain OU, for oculus uterque , which means both eyes. Farsighted people see clearly at a distance, but things up close are blurry.

Nearsighted people see things clearly near, but things at a distance are blurry. Next to either a plus or minus sign are numbers.

These are diopters , and they are the unit of measure for the correction , or focusing power of the lenses your eyes require.

Diopter is often abbreviated with a "D". The farther the numbers are from 0, the worse is your eyesight, and the more vision correction you will need. The numbers are in. Someone with Astigmatism arises out of the cornea being shaped more like a football than a basketball.

If you have astigmatism, your prescription will be written in the form: S x C x Axis , where "S" is the "spherical", or power, portion of your prescription, which we described above. It can be a negative or a positive number, and the higher the number, the more astigmatism you have.

Axis is a measure in degrees, and will be between 0 and degrees. It refers to the orientation of your astigmatism. Two examples of a prescription with astigmatism are:. Person One: Parents know they need to protect their children's skin from harmful sun rays. But many forget that the eyes need to be protected, too. Sun exposure may set children up for possible vision problems later in life. The sun can cause sunburned corneas, cancer of the eyelid, cataracts, and macular degeneration, among other problems.

Children also tend to spend more time outdoors than their parents, often in places where there is a lot of sun reflection. These include beaches, pools, and amusement parks. It is also important for your child to wear sunglasses in the snow as well.

Most UV eye damage is the result of years of exposure. Buy your child sunglasses that block both kinds of UV rays. Make sure the sunglasses fit correctly and are comfortable. Search Encyclopedia. Eyeglasses and Contact Lenses Lenses for correcting or improving vision There are 2 types of lenses prescribed for correcting or improving vision. These include: Eyeglasses. How to read an eyeglass prescription The lens power of eyeglasses is measured in diopters.

When looking at an eyeglass prescription, you will see the following abbreviations: OD Oculus dexterous. The eyeglass prescription may also have these measurements: Sphere. What are the different types of eyeglass lenses? The type of lenses used in eyeglasses depends on the type of vision problem, and may include: Concave lenses.

Eyeglasses for children If your child is old enough, let them play an active role in choosing their own glasses. Think about the features below when buying eyeglasses for children: Shatterproof and impact-resistant lenses, especially for children who play sports Scratch-resistant coating on the lenses Spring-loaded frames that are less likely to be bent or warped Silicone nose pads that prevent glasses from slipping Ear pieces that wrap around the ear cable temples are advised for children under 4 years.

Facts about contact lenses About 45 million Americans wear contact lenses. In general, there are 2 types of contact lenses: Rigid, gas-permeable lenses Soft, water-absorbing lenses Reading a contact lens prescription A contact lens prescription includes more information than an eyeglass prescription. The contact lens prescription often includes: Contact lens power measured in diopters, like eyeglasses Contact lens base curve Contact lens diameter Eye care specialists are required by federal law to give you a copy of your contact lens specifications.

Protect your child's eyes from the sun Parents know they need to protect their children's skin from harmful sun rays.