Normal eye (Emmetropia)
Eyes that require no glasses or contacts to see clearly are called emmetropic, that is, they have no refractive error. The major factors of the eye anatomy which determine if glasses will be needed are the cornea, the crystalline lens within the eye and the distance to the retina (called the 'axial length' of the eye). When an eye sees clearest without any eyeglass correction, it means that the corneal lens power is just right for the length of the eye. Imbalances in these features lead to refractive errors, such as far-sightedness and near-sightedness. Astigmatism results from a different shape of the cornea and presbyopia from gradual hardening of the crystalline lens within the eye. We cannot change the length of the eye and, short of lens removal, cannot alter the power of the crystalline lens. We are able to effectively alter the curvature of the cornea to affect the eye's refractive imbalances to obtain clearer vision without glasses. Refractive procedures such as LASIK and PRK are designed to do just that.
People with farsightedness usually have healthy eyes, but they experience problems with the way light is focused. When the cornea is too flat for the length of the eye, the best focus point falls behind the retina. The image formed on the retina itself is therefore blurry unless the eye alters focus of the crystalline lens within the eye to clear up the vision. In our thirties and forties we lose some of that ability and the near vision first becomes blurred, requiring reading glasses. In the forties and fifties, the eye can no longer focus for distance either and the hyperopic patient finds themselves in full time distance and near glasses.
Refractive procedures such as LASIK can steepen the central cornea to increase its power to bring light into focus again on the retina. This is welcome news to far-sighted patients who have enjoyed a life without glasses, who find their near and distance vision worsening as they reach the forties.
In the common condition of myopia or near-sightedness, the eye grows too long and the cornea is too steep to bring clear focus to the retina. Light rays focus most clearly within the eye in front of the retina. The rays that reach the retina are therefore blurred. Myopic patients usually find themselves using glasses for distance tasks such as driving and watching television. If the degree of myopia is not severe, they may be able to read without glasses, since their natural focus point is closer than normal. It is possible to flatten the cornea with refractive procedures such as LASIK to reduce or eliminate the reliance on glasses.
If the cornea is more cylindrical than round in shape, the result is blurred vision which can be corrected with astigmatic lenses. An eye can be astigmatic alone or in combination with myopia, hyperopia and/or presbyopia.
Presbyopia is the condition which occurs in all eyes gradually throughout life. As the crystalline lens within the eye loses elasticity with years, it loses ability to change focus from far to near and back. Even people who require no glasses still must change focus from far to near by 'accomodating' or changing the power by contraction of tiny muscle fibers around the lens inside the eye. As we lose this ability (usually in the early forties), we start to use reading glasses, or add a bifocal to our existing glasses, or if moderately near-sighted, we simply remove the glasses to read. Hyperopic patients are often most frustrated with this process, because after a lifetime of no glasses, they find the near vision deteriorating, then the distance vision worsens as well.
Monovision is an attempt to create a balance that would allow someone beyond the forties to see clearly at distance and near without any glasses. One eye is corrected for distance clarity and the other for near. The trade-off is that the distance eye will be somewhat blurred at near points and vise-versa. People who can tolerate the modest visual imbalance do perform most daily tasks without any eyeglass correction well beyond the forties. It is possible to induce this state of monovision with contact lenses or refractive surgery.