The eye is a complicated organ, with many components working together as a unit. It changes over time.  Understanding your own vision can help you make good decisions.

When light enters the eye, it passes through the cornea (a clear, rounded surface in front of the iris) and travels to the lens (a clear, flexible surface behind the pupil).  The cornea and the lens refract (bend) the light, so that it will focus on the retina, the eye's inside back wall.  (A small muscle around the lens helps the lens change shape.  If the light is coming from a closer object, the muscle squeezes the lens, increasing its curvature.  If the light is coming from a distant object, the muscle relaxes, lessening its curve.  These changes in shape change the focusing power of the lens, that is, the way the lens bends the light.)

If everything is normal, the light rays are focused (come together) onto the retina.  The retina consists of millions of visual cells.  They convert the light into electrical impulses and send those impulses through the optic nerve to the brain. As a result, we see a clear, sharp image, with everything – both near and far objects – in focus. 

But if the eyeball is too long, or the cornea is overly curved, the light rays will be improperly focused.  They will come together in front of the retina.  Then, close objects will appear sharp, but distant ones will be blurry.  This condition is, of course, known as nearsightedness or myopia. 

If the eyeball is too short, or the cornea has less of a curve than it should, the opposite happens.  The light focuses on a spot behind the retina. Near objects appear blurry; more distant ones are sharp.  This condition is farsightedness or hyperopia.  

A third possibility is that the cornea (or, much less often, the lens) is unevenly curved.   The result is that light may focus on the retina, but not on one focal point. This causes a blurry image, a condition known as astigmatism.  A person can have astigmatism alone, or combined with either nearsightedness or farsightedness.

Despite what you may be been told, reading in bad light, squinting, and sitting close to the television will not cause or worsen any of these three conditions (though they may strain your eyes).  They are known as refractive errors because they involve problems with the way the eyes refract light.  There are many theories about what causes them, but in general people do seem to be born with them (though astigmatism can develop after an illness or surgery of the eye, or an accident).  Often they are first noticed in childhood, worsen through adolescence, then become stable.

Refractive errors are the most common eye conditions and have been traditionally treated by wearing glasses (technically called corrective lenses) and contact lenses. Today there are several types of contact lenses: hard, soft, rigid gas-permeable, disposable, extended wear, etc. You should discuss these options with your eye doctor and understand what's best for you before making any choices. 

These three conditions are also the ones that may be alleviated by refractive surgery.  This surgery uses lasers to change the shape of the cornea, which then changes how the cornea refracts light.   But there is a fourth refractive error, known as presbyopia that is generally not correctable by refractive surgery.

Presbyopia occurs naturally as we get older.  Unfortunately, in this context "older" can mean as young as forty.  That's when we may start to notice that we're holding what we read further from our eyes.  We're losing the ability to focus on closer objects because the lenses in our eyes are becoming less flexible.  Despite the action of the muscle surrounding it, the lens changes shape less easily.  So even if we've never had glasses before, we now need "reading glasses."

Various types of glasses can compensate for presbyopia, ranging from simple, over-the-counter magnifiers (no need to spend $10.00 or more for them at the pharmacy; you can get perfectly good ones at a $1.00 store) to prescription lenses to contact lens.  If you already wear glasses, then you'll need either a separate pair of glasses for close vision (reading glasses) or bifocals. 

There are many types of bifocals available today: traditional (with a visible line), lineless (often called progressive), trifocal (also called multifocal, and available with or without lines).  Bifocal contact lenses are also available, as well as monovision contacts, in which one eye uses a lens for distance vision, and the other eye uses one for a close vision; a more recent option is modified monovision: a bifocal contact in one eye, a lens for distance vision in the other.  Again, you need to discuss the options with your eye doctor and understand what's best for you before making any choices.

However, remember that refractive surgery generally has no effect on presbyopia.  If you wore reading glasses before the surgery, you'll still need some form of them after it.