Understanding Common Focusing Errors

Understanding Common Focusing Errors

By Dan Zimmer

Focusing errors fall under four categories: Myopia, Astigmatism, Hyperopia, and Presbyopia.  Below is a guide to understanding the characteristics of each of these optical conditions.

Myopia:

Myopia is the medical term for nearsightedness, a condition that affects more than 70 million people in North America.  It occurs when the eye is too long for the curvature of the cornea.  As a result, light rays that enter the eye do not come into sharp focus on the retina which is at the back of the eye, and instead they focus further forward.  The result of this skewed focus is blurred vision.

The term ‘nearsighted’ means that one can see ‘near’ objects clearly without glasses or contact lenses, but objects that are further away are blurry.  The condition occurs in varying degrees.  The more myopic  a person is, the blurrier the appearance of distant objects, the higher the eyeglass prescription, and the thicker the lenses.

Of all myopic people, about 90% have corrections that are less than -6.00 diopters.   Below shows the categories of myopia severity:

Mild Myopia                                       < -3.00 diopters

Moderate Myopia                                    -3.00 to -6.00 diopters

Severe Myopia                                        -6.00 to -9.00 diopters

Extreme Myopia                                  > -9.00 diopters

Astigmatism:

Many myopic patients have a degree of astigmatism.  In fact, it is the most common refractive condition.  Astigmatism occurs when the eye is oval shaped instead of being a perfect sphere.   As a result, patients with astigmatism experience distorted or tilted images as a result of the unequal bending of the light that enters the eye.  Patients that have high degrees of astigmatism have blurred vision for both near and far objects.

Below is a list of the categories of severity of astigmatism:

                                                      Mild Astigmatism                           < 1.00 diopters

                                                      Moderate Astigmatism                       1.00 to 2.00 diopters

                                                      Severe Astigmatism                           2.00 to 3.00 diopters

                                                      Extreme Astigmatism                      > 3.00 diopters

Hyperopia:

Hyperopia is the medical term for ‘farsightedness’ and it occurs when the eye is too short for the corneal curvature.  Light rays focus behind the retina, which produces a blurred image.  Farsighted individuals can use their focusing muscles to move the image forward into the retina, but distant objects are seen more clearly than near objects.  Laser technologies, like LASIK, can correct this condition.

Presbyopia:

Presbyopia is part of the normal aging process, where the lens of the eye loses some of its flexibility.  The condition usually occurs between the ages of 40 and 50, and everyone experiences some degree of presbyopia.  As a result, many nearsighted people begin wearing bifocals in the forties, and people who have never worn glasses may begin to need reading glasses.

An advantage of mild myopia is the ability to stop wearing glasses after age 40 as the myopia counteracts the presbyopia.  The downfall of LASIK is that it causes patients to lose this ability.  As LASIK produces normal sight in individuals, LASIK patients begin to need reading glasses like any other normally sighted person would, when they reach the age of 40 or 50. 

 

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