Nearsightedness, Farsightedness, and Astigmatism - How Does it All Work?
We've all heard the terms nearsightedness, farsightedness, and the dreaded astigmatism. If you wear glasses or know someone that wears glasses, chances are, you've identified with one of the terms. But what exactly do they mean? And those words myopia and hyperopia, how do they fit into the scheme of things? Well, it is time to find out!
Myopia, more commonly known as nearsightedness, describes an eye that can see clearly up close but is blurry when looking in the distance. This can occur because the eyeball is too long or the curvature of cornea is too steep. The result of one or both of these causes is that light focuses in front of the retina.
In order to push light backwards so it lands on the retina, we must use a minus powered, concave lens. Thus, an individual with myopia will use a minus lens.
In contrast to myopia, hyperopia or farsightedness, describes an eye that can see clearly in the distance, but has difficulty seeing up close. Note that in the case of hyperopia, the individual does not necessarily have blurry vision up close, rather they have difficulty which can manifest as eye strain, fatigue, or even headaches.
Hyperopia occurs because the eyeball is too short or the curvature of the cornea is too flat. This causes light to focus behind the retina.
Unlike myopia, in hyperopia, the eye can actually bring light from behind the retina and focus it on the retina. This involves using natural lens of the eye in a process called accommodation. The issues that arises is that the natural lens begins working much hard and much more than it should. This in turn leads to eye strain and fatigue.
In order to move light forward so it lands on the retina, we must us a plus powered, convex lens. Thus, an individual with hyperopia will use a plus lens.
Astigmatism represents a condition where the eye takes on a more oval (or cylindrical) shape rather than a spherical shape. The classic metaphor is the comparison between a more oval shaped football and a more sphere shaped basketball. Astigmatism is most often due to curvature of the cornea (front of the eye), but may also be due to the natural lens of the eye as well. Having an astigmatic eye means that two meridians must be corrected. It means that instead of just one point of light not focusing on the retina, we have two that are not focused on the retina.
Ok Dr. Neufeld, you lost me.
This is where astigmatism gets a little complicated. With astigmatism, an axis will occur. This axis will determine where the two meridians lie and essentially acts as the split between the two meridians. Hence, if you have a prescription with astigmatism, you will see the "axis" as the third number on the script.
Each meridian, contains a different prescription. Thus, you can imagine an eye with astigmatism as almost having two prescriptions in one. The key to understanding astigmatism once again lies in our theme of focusing light on the retina.
In order to correct astigmatism, we must use a cylindrical lens. This enables us to focus light on the retina in both meridians.
Now that we have a basic understanding of the three types of refractive errors, it is time to delve into another condition that universally affects every individual around the mid-40s -- presbyopia. Tune in next week to learn more!
-Dr. Aaron Neufeld
Dr. Aaron Neufeld is the Chief Optometrist at Los Altos Optometric Group and primary author and editor of The EYE Digest.
Contact him with questions or ideas for future articles at (650) 948-3700 or firstname.lastname@example.org