Correcting Optical Errors

By Michael J. Dodd, MD

Maryland Eye Associates

. http://marylandeyeassociates.com

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Correcting Optical Errors

Optics is usually considered a dull boring subject, even by most ophthalmologists. It is however, very important to patients. In this brief overview we will discuss some of the common terms used in optics and try to explain them in common sense, non-technical terms. We will also discuss some clinical conditions and their effect on optics.
For our purposes, optics is defined as the bending effect of light rays by means of lenses.
We will start our discussion with one of the most common optical errors, nearsightedness. Myopia means the same as nearsightedness. Myopic patients can see objects up close but not in distance (the British call myopia “shortsightedness”). Myopic patients need to wear glasses or contact lenses to see clearly at distance. The myopic lenses are thick in the periphery, but thin in the center (concave lenses) and tend to make images slightly smaller. Today LASIK surgery can correct myopia by flattening the central cornea with great precision.
Interestingly, as people age and develop nuclear sclerotic cataracts (the most common type) the optics of the eye change toward more myopia. Hence, you may hear elderly people claim that they can read without their glasses (“second sight”). This actually means they are developing nuclear cataracts. Once cataract and lens implant surgery is performed, the patients can typically see well for distance without glasses (emmetropia).
This leads to the next optical error, presbyopia, another very common condition. In fact, presbyopia is a process, which begins in the mid-forties and extends throughout ones life. So virtually, all human beings over 45 experience this condition. The term is Latin for “old eyes.” To understand this, one must understand that all eyes have a natural lens. In a child, it is clear and about the size of an M&M candy. The lens functions to focus distance and near by getting slightly thinner and thicker.
As we age, the elasticity of the lens is lost and by age 45 it can only focus close at about 20 inches. Since this is just beyond our human reading distance we lose the ability to see up close and need magnifiers to bring the image into range. Incidentally, as the lens continues to lose elasticity it also changes color and as the lens cells dies, small flecks and spots accumulate. This is the beginning of a cataract (Latin waterfall).
Astigmatism (Greek “a” = not; stigma = “point”) is the next optical error we will discuss. Many patients think astigmatism is an eye disease, which it is not. It is simply a classification of optical error. It can be best explained by the following exmple assume you have a clear basketball which you cut in half and place on a table. If you hold a light above it, the light rays will converge and focus on the table as a point. If you squeeze the ball in with both hands to make it slightly oval on the table surface, the light will now focus in the shape of a line, not a point. This is called a cylinder lens and light rays that pass through it cannot focus on a point.
A patient with astigmatism has a cylinder lens shape in his cornea. They cannot see clearly at distance or near. To “fix” this the optician grinds a cylinder lens in spectacles with the rotation of the axis 90 from that in the patients cornea. This gives the patient sharp vision. Contact lenses and LASIK surgery can also treat astigmatism.
The last type of optical error
is farsightedness or hyperopia. This
is the opposite of myopia. Typically, patients see better at far than at near. There are however, patients with
extreme hyperopia who also see poorly at distance. Glasses and contact lenses correct hyperopia well. Hyperopic lenses are thick centrally and thin in the periphery (convex lenses) and tend to magnify images. LASIK will correct small degrees of hyperopia only.
We hope you have enjoyed our brief discussion of optics.