Let me know what you think about this section.  Refractive errors are really quite difficult to understand, but if you are considering any type of refractive surgery, it is essential that you have at least some knowledge of your refractive error, and how presbyopia will affect you as you age.  Keep in mind that once you have had refractive surgery, your eyes will be emmetropic, unless you elect to have monovision.

Emmetropia (Normal distance vision)  and Presbyopia (Aging)*

The reason emmetropes can see clearly at near while they are young is because the lens of the eye can change its shape so that it thickens and provides extra magnifying power.  The process whereby the lens thickens is called accommodation, and prior to age 42 we accommodate automatically, effortlessly.  Then this horrible thing happens and we start to loose our ability to accommodate in process is called Presbyopia.  As normal folks become presbyopic they loose their ability to see clearly up close (decreased near vision, arms getting too short, etc...).  EVERYBODY becomes presbyopic at about age 42, and presbyopia worsens slowly up to the age of about 65.  After you reach age 65 your ability to accommodate is completely shot.  Presbyopia happens to everyone (myopes, hyperopes, astigmats, and emmetropes) BUT each type of refractive error responds differently as presbyopia takes it's toll.

Myopia  (Nearsighted) and Presbyopia (Aging)*

As a myope ages he looses the focusing power of his lens (becomes presbyopic) just like everyone else.  However, a myope has a built in ability to see clearly at near as soon as he removes his glasses or contacts.  If you wear glasses, have simple nearsightedness (that is, not too much astigmatism), and are over the age of 42, you may have noticed that you need to remove your glasses to see clearly up close.  Once you put your glasses back on, your eyes behave just like normal folks (emmetropes), and if you want to read through your glasses, you will need extra magnifying power, just like emmetropes.  Since you already wear glasses, now you are going to need a bifocal lens.  Having refractive surgery, wearing glasses, or wearing contacts all has the same effect on your eyes, optically speaking.  All these methods effectively make you into an emmetrope.
 

Hyperopia (Farsightedness) and Presbyopia (Aging)*

Now's here is the catch.  Before age 40, you might be farsighted without you even knowing it.  That is because accommodation can overcome farsightedness.  A high hyperope probably started wearing glasses as a child.  But moderate and low hyperopes typically don't wear any glasses until they reach their late thirties.  Because some of their accommodative power is "used up" to clear up their distance vision, they need to wear reading glasses at an earlier age, usually late thirties.  By their late forties they usually need to wear glasses for working on the computer, and sometimes glasses for the distance, and eventually they end up wearing trifocal glasses all the time, for distance, intermediate vision, and especially for near vision.
 

Astigmatism

So, an astigmat has an eye shaped like the bottom of a spoon, with two different degrees of curvature to his cornea.  Now, in a person who is really nearsighted with astigmatism, effectively his eye is less nearsighted in one direction (say up and down) than another (side to side).  Optically light always focuses in front of the retina, but some light rays focus further in front than other.  Similarly a farsighted person would be more farsighted in one "direction" than in another.  In farsighted people distant light rays always focus behind the retina, and some focus further behind than others.  Just to make things really messy, there are actually people out there with so called "complex astigmatism."  In these people, some distant light rays focus in front of the retina, and some focus behind the retina.  In a way, these people are both nearsighted and farsighted at the same time.

Irregular astigmatism usually refers to people with people who have an irregular cornea surface.  In these unfortunate chaps light rays entering the eye scatter about in various directions, and cannot be focused clearly using lenses.  Sometimes irregular astigmatism can be treated by wearing contact lenses, particularly gas permeable (hard) lenses

Accommodation

Consider for a moment a 20 year old fighter pilot with perfect distance vision.  While gazing off towards the horizon his eyes are in their "resting position" and we say that his accommodation is relaxed.  Suddenly a light goes off on his instrument panel about 30 inches away, and he looks directly at the light.  Anyone who has used binoculars, a telescope, or a camera, or in fact any optical system knows that in order to do this, you must refocus.  The eye also has to refocus, and it does so by a process called accommodation.  During accommodation, a band of circular muscles within the cilliary body contracts.  This releases tension on the lens zonules, and the lens expands in the center, thereby becoming more round, thereby giving it more magnifying power.

Image the lens in the shape of the planet Saturn filled with a soft jelly, and the cilliary body as the planet's rings.  The rings are attached to the equator of the lens (planet surface) by thousands and thousands of strings.  Expanding the rings puts tension on the lens equator, and since the lens is soft and jelly like, pulling on the equator flattens out the planet (biconvex shape).  In its resting state, the rings are pulled outward, stretching the equator of the lens and flattening the poles.  Contraction of the cilliary muscle is similar to contraction of the rings which loosens tension on the strings and allow the jelly to become more spherical and therefore a smaller radius of curvature at the poles.

OK one last time:  When our pilot tries to see something up close (less than 6 feet away), a circular band of muscles in the cilliary body (like the rings of saturn) contracts.  This loosens tension on the lens zonules (strings) causing the balloon to look less like a magnifying lens (thick in the center, thin on the edges) and more like a sphere (Saturn).  Are you confused yet?

The really important point of all this is that the system breaks down as we age.  There is controversy over exactly how this happens, but the simplest explanation is that our jelly like lens hardens, and can't flex as well as it should

*  You may be thinking, Hey!  That was really rude putting  "aging" in here.  I just wanted to learn
    about eyes. SORRY!! Get over it!  We are all getting older, we all get presbyopia, and we
   all get cataracts........

**  Optically astigmatism does not necessarily come from the cornea, but can also come from
     uneven curvature of the surface of the lens, or other abnormalities within the eye.
 
 

This page last updated 04/25/2001