Patient guide
How your eyes focus, and how to look after them
By Dr. Sam Khoury, Optometrist10 min read
Glasses do not fix sudden changes. Call the same day for sudden blurred or lost vision, a shadow or curtain across your vision, new flashes or floaters, eye pain, or a red eye that is sensitive to light. A shadow, a curtain or sudden vision loss should not wait for office hours at all. You can contact the office at (609) 246-0236. If the office is closed, or you cannot reach us, go to a hospital emergency room — most have an ophthalmologist on call. Vision loss together with weakness, numbness or trouble speaking is a 911 call.
Most people who come in for an eye exam have healthy eyes that simply do not focus perfectly. That is not a disease. It is a question of shape: how long the eye is, and how curved its front surface is. This guide explains the four common focusing problems, how to read your glasses prescription, and the everyday habits that look after your eyes for the long run.
How the eye focuses
Light enters through the cornea, the clear dome at the front of the eye, and then passes through the lens just behind the pupil. Together they bend the light so that it comes to a sharp point on the retina, the light-sensitive layer lining the back of the eye. The retina turns that image into signals the brain reads as sight.
For a sharp image, the focusing power has to match the length of the eye. A very small mismatch is enough to blur things, which is why focusing problems are so common. Their medical name is refractive errors; refraction simply means the bending of light.
In focus. The light comes to a point on the retina.
Nearsighted. The eye is a little long, so the light comes to a point in front of the retina and is blurred again by the time it arrives.
Farsighted. The eye is a little short, so the light reaches the retina before it has come to a point.
Glasses and contact lenses add or take away a little focusing power so the light lands where it should. They do not change the eye itself.
The four common focusing problems
Nearsightedness (myopia)
Close things are clear and distant things blur: road signs, the board at school, faces across a room. The eye is usually a little too long. Nearsightedness typically starts in childhood and increases while the eye is still growing, then tends to settle in adulthood. In children there are now ways to slow it down; see myopia management.
A strongly nearsighted eye is also a longer, more stretched eye, and that raises the risk of a retinal detachment later in life. It is worth knowing the warning signs.
Farsightedness (hyperopia)
The eye is a little too short. Young eyes can often make up the difference by focusing harder, so a farsighted child or young adult may read the distance chart perfectly and still get tired eyes, headaches or swimming words after reading. Children rarely complain, because they assume everyone sees that way. As that extra focusing ability fades with age, farsightedness shows up as blur, first up close and later in the distance too.
In a child, a large amount of farsightedness or astigmatism, or a big difference between the two eyes, can keep one eye from developing full sharpness even with glasses later. See lazy eye (amblyopia).
Astigmatism
The front of the eye is curved more steeply in one direction than the other, shaped more like the back of a spoon than a ball. Light focuses at two places instead of one, so things blur at every distance: letters look smeared or shadowed, and lights at night can look streaky. A small amount is normal and very common, and it often comes along with nearsightedness or farsightedness. Glasses correct it, and so do contact lenses made for astigmatism.
Needing reading glasses (presbyopia)
To focus up close, the lens inside the eye changes shape, and with age it slowly stiffens. Most people notice it sometime in their forties: small print needs better light, and the phone drifts farther away. It happens to everyone, including people who have never needed glasses. Reading glasses, bifocals, progressive lenses and multifocal contact lenses all solve it in different ways; which suits you depends on your work, your hobbies and your distance prescription.
Reading your glasses prescription
A glasses prescription is a short list of numbers for each eye. This is what they mean.
- OD, OS, OU
- Right eye, left eye and both eyes, from the Latin. Some prescriptions simply say R and L.
- Sphere (SPH)
- The main focusing power, in diopters. A minus number means nearsighted and a plus number means farsighted. The farther from zero, the stronger the lens.
- Cylinder (CYL)
- The extra power that corrects astigmatism. Blank or zero means little or none.
- Axis
- The direction of the astigmatism correction, from 1 to 180 degrees. It is a direction, not a strength, so a bigger number is not worse.
- Add
- Extra power for reading, used in bifocals and progressive lenses. It usually appears from the forties on.
What 20/20 means
20/20 means you can read at 20 feet what a typical eye reads at 20 feet. At 20/40, you need to stand at 20 feet to read what a typical eye can read from 40. It describes how sharply you see at a distance; it says nothing about how healthy the eye is.
A contact lens prescription is different
It names the lens brand, its curve and its size, which come from a fitting on your eye. A glasses prescription cannot be used to order contact lenses.
Glasses, contact lenses and LASIK
Glasses are the simplest correction and the easiest to change. For long hours at a computer, a pair set for that distance can be more comfortable than all-purpose lenses.
Contact lenses give a wider field of view and suit sports. They need a fitting and good habits: do not sleep in them unless you were told your lenses are made for it, keep them away from water, including showers, pools and tap water, and replace them on schedule. If you wear lenses and an eye becomes red and painful, take the lens out and call now.
LASIK. Whether laser correction suits you depends on your prescription, corneal shape and tear film, all measured here. The procedure is performed by an ophthalmic surgeon; the assessment before and the follow-up after are done in this office. A prescription that is still changing is a reason to wait.
Looking after your eyes
None of this is complicated. Most of it is the same advice that looks after the rest of you.
- Have your eyes examined, even when you see well Some eye diseases, glaucoma among them, cause no symptoms until vision has already been lost. An eye exam checks the health of the eye as well as the prescription; a vision screening or an online test only checks how well you see. The American Optometric Association advises an exam at least every two years for adults up to 64, every year from 65, and every year for school-age children, and more often if you have diabetes, a strong prescription or a family history of eye disease.
- Wear sunglasses that block UV Years of sunlight are linked to cataracts and to growths on the white of the eye. Look for a label that says 100 percent UV protection or UV400. Darkness alone tells you nothing, and price tells you nothing either. A brimmed hat helps, and children need protection as much as adults.
- Do not smoke Smoking raises the risk of macular degeneration and of cataracts, two of the main causes of vision loss with age. Stopping lowers the risk.
- Eat for your eyes the ordinary way Leafy greens, colorful vegetables and fish, as part of a generally healthy diet, are linked with healthier eyes later in life. Eye vitamins are a different matter: the formula studied for macular degeneration helps people with certain stages of it, and has not been shown to protect healthy eyes.
- Keep blood sugar and blood pressure in check Both affect the small blood vessels of the retina. If you have diabetes, a dilated eye exam is part of your diabetes care, usually once a year, even when your vision seems fine.
- Protect your eyes from injury Wear safety glasses for power tools, mowing and trimming, and anything involving chemicals, including strong cleaning products. Everyday glasses are not safety glasses unless they are marked Z87+. Wear sports goggles for racquet sports and basketball. If a chemical splashes into your eye, rinse it with running water right away for at least 15 minutes, then have it checked the same day.
- Give your eyes screen breaks Screens do not damage the eyes, but people blink less while looking at them, which leaves eyes dry and tired. Every 20 minutes, look at something 20 feet away for 20 seconds and blink fully. If your eyes stay tired, the dry eye guide is the next place to look.
- Send children outside Time in bright daylight is the most consistently supported protection against children becoming nearsighted. More on slowing nearsightedness.
Common questions
Will wearing glasses make my eyes weaker?
No. Glasses focus the light; they do not change the eye. Vision without them can seem worse once you are used to seeing clearly, but the eye itself has not changed. Children's nearsightedness tends to increase while they are growing, with or without glasses, which is a reason to look at myopia management, not a reason to avoid glasses.
Does reading in dim light or sitting close to a screen damage the eyes?
No. It can make the eyes tired, but it does not cause lasting harm. A child who keeps sitting very close to the TV may be nearsighted, though, and is worth having checked.
Can carrots or eye exercises improve my eyesight?
No. Vitamin A matters for vision, but a normal diet supplies enough, and more will not change a prescription. Exercises cannot change the length of the eye or the curve of the cornea. Some exercises do help specific eye-teaming problems, which an exam can identify, but they do not remove the need for glasses.
Do I need blue-light glasses?
The evidence does not show that blue-light-filtering lenses reduce eye strain from screens, and the light from screens is not known to damage the eyes. Screen breaks and full blinks do more.
Are drugstore reading glasses all right?
For many people, yes, when both eyes need about the same power and there is little astigmatism. They are not right for everyone, and they do not replace an exam, which also checks the health of the eye.
My vision changed and I did not do anything different. Why?
Prescriptions drift, especially while children are growing and again from the forties on. But vision that changes quickly, changes in one eye only, or swings from day to day deserves a call rather than a new pair of glasses. Changes in blood sugar, for example, can shift focus for a while.
Does a routine eye exam go through my vision plan or my medical insurance?
A routine exam for glasses or contact lenses usually goes through a vision plan. If you are coming in about a symptom or a condition, it is a medical visit and goes through your medical insurance. Call with your plan details and it can be sorted out before your visit rather than after.
Book an eye exam
A routine exam checks your prescription and the health of each eye at the same visit.
Book an exam Call (609) 246-0236
Khoury Eye Care, Hamilton Mall, 4403 Black Horse Pike, Mays Landing.