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Kornmehl Laser Eye Associates

Laser Vision Correction, Dry Eye Treatment, Cataract Surgery,
Contact Lenses & General Eyecare

How the Human Eye Works Part 3

Most people who have had an eye exam that includes a test to measure visual acuity, clarity or sharpness of vision, recognize the simple notation 20/20 as meaning “normal vision.”  What do those numbers mean?  Let’s say your vision is 20/40.  That means you can see at twenty feet what a person with normal vision can see at forty feet.  Your measure of visual acuity is determined by using the Snellen chart, that familiar eye chart with progressively smaller letters on each line.  Although it is considered an accurate vision test, its results are sometimes affected by such variables as squinting, guessing at the letters, and room light.

So, numbers such as 20/20 or 20/40 describe your visual acuity but do not measure your refractive error- how accurately your eye bends, or refracts, light.  When an eye doctor measures your refractive error, what you end up with is your eyeglass prescription.  Finding an eye doctor whose measurements are impeccable is crucial, not just for your eyeglass prescription but also, as you will learn later, for laser vision correction.

Your eyeglass prescription is written in numbers.  The type and degree of refractive error is quantified in units of measure called diopters.  If you have ever wondered what those numbers mean, here is how to read and understand your prescription.

To arrive at your prescription, your doctor takes three measurements during the eye exam: sphere, cylinder, and axis.  Your prescription for glasses may look something like this:

OD  -1.25   —-    —-

OS  -1.25  -.25  x 170

OD and OS refer to the right and left eyes, respectively.  The first number next to OD or OS represents the sphere.  The sphere measure tells the eye doctor where your eye focuses light: on the retina (normal vision), in front of the retina (myopia), or behind the retina (hyperopia).  In other words, the sphere measure reveals whether you are nearsighted or farsighted.  A negative diopter indicates myopia, or nearsightedness.  A positive diopter indicates hyperopia, or farsightedness.  The higher the number, the stronger the prescription.  In the example above, the person has mild myopia (-1.25 diopters) in both eyes.

The number in the second column represents the cylinder.  The cylinder measure indicates whether or not the patient has astigmatism.  If the cylinder column is not blank, you have some degree of astigmatism.  The larger the number, the more astigmatism you have.  The example above reveals that this person has no astigmatism in the right eye, and a small amount (-.25 diopter) in the left eye.

If astigmatism is present, your eye doctor takes an axis measurement.  The axis measure indicates where irregularity lies on the eyeball.  In the prescription above, the astigmatism in the left eye is positioned at the 170-degree axis.

Eyeglasses have been around for hundreds of years.  As early as the thirteenth century, inventors in China and Europe inserted magnifiers into frames, making the first prototype for our modern-day eyeglasses.  Like the early versions, today’s eyeglasses work like magnifying glasses that enhance the eye’s ability to focus sharply, whether near or far.  The amount of curvature in the spectacle lens determines how light bends before it reaches your cornea.  Vision is corrected, depending on the angle of refraction, to compensate for your focusing error.

Eyeglasses have a number of advantages.  They are usually affordable, are easy to maintain, and can be adapted for a number of different uses, such as reading, active sports, and driving.  They also have disadvantages.  Eyeglasses may restrict peripheral vision, the outer part of your field of vision; prove difficult in certain weather conditions, such as rain or snow; and make images appear smaller or larger than they really are.  They may cause a number of visual aberrations, including halos around lights, and the lenses usually need to be replaced as your vision changes.  Eyeglasses may interfere with certain occupations and recreational activities- swimming, for example.  And some people just don’t like the way they look in glasses.

Contact lenses offer another option for correcting vision.  Like eyeglasses, they make up the difference between the amount of refraction your eye can accomplish on its own and what is needed for sharp focus.  Because they are extremely thin and are custom-shaped for your cornea, contact lenses float on the surface of your eye; they are held in place by natural suction and are constantly lubricated by the eye’s own moisture.

Contact lenses have some advantages over eyeglasses.  For example, contacts enable the wearer to have more natural vision (including better peripheral vision), cause little noticeable change in cosmetic appearance, and allow more freedom in recreational activities.  On the other hand, contacts may require maintenance- continuous, frequent cleaning.  Users must buy cleaning and storage solutions.  The lenses may tear easily.  They may be inconvenient for traveling, and also are easily lost.  Contacts may be uncomfortable for patients with dry eyes or for those who live and work in polluted city air.  They may cause visual aberrations (including halos and uneven vision) and always carry an increased risk of infection and possible corneal scarring.  Individuals who live in higher altitudes may become intolerant of contact lenses over time because of the air’s lower oxygen and humidity content.

The variety of contact lenses available today is dazzling.  Costs for contacts vary widely, depending on the type you need.

Orthokeratology, is a technique for treating myopia, or nearsightedness.  It uses a series of rigid contact lenses that apply pressure to the cornea to flatten it.  The effects are not permanent and require continued dependence on daily-wear maintenance lenses to retain the reshaping.  Orthokeratology is generally only effective, even temporarily, for low levels of nearsightedness.  The technique is expensive and high maintenance and requires continuous follow-up visits.  Long-term effects can include permanently warped corneas.  The risk of infection may also be greater than that from normal contact lens wear.

Interested in LASIK in Boston, visit Kornmehl Laser Eye Associates for a consultation.