Myopia (near-sightedness)
Myopia or near-sightedness is where the power or refractive strength of the eye is too strong for its length. This means that light from a far distance will focus in front of the retina. Because of this it is easier to see objects at a near distance than at a far distance. For mild amounts of myopia this means that objects within arms length are in good focus, however, for high amounts of myopia only objects within inches of the eye can be seen clearly.
To correct for myopia a minus lens is placed in front of the eye to diverge light so that it enters the eye correctly to focus further back on the retina. Contact lenses do the same, but they are placed on the eye itself. LASIK removes tissue on the cornea to flatten its shape and moves the focal point of the eye further back to focus properly on the retina.
Hyperopia (far-sightedness)
Hyperopia or far-sightedness is where the power or refractive strength of the eye is too weak for its length. This means that light from a far distance will focus behind the retina. Because of this it is easier to see at a far distance than at a near distance. Mild amounts of hyperopia at a young age can easily be overcome by the strong focal ability of the crystalline lens inside the eye. However, at an older age mild amounts of hyperopia cannot be overcome by the weak focal ability of the crystalline lens and this makes visual acuity blurry far away and even worse at near. High amounts of hyperopia at a young age may cause significant visual strain, headaches, avoidance of near work or an eye turn. At an older age high amounts of hyperopia cause very blurry distant visual acuity and extremely blurry near visual acuity.
To correct for hyperopia a plus lens is placed in front of the eye to converge light so that it enters the eye correctly to focus closer forward on the retina. Contact lenses do the same, but they are placed on the eye itself. LASIK removes tissue on the cornea to steepen its shape and moves the focal point further forward to focus properly on the retina.
Astigmatism
Astigmatism is where the eye is shaped like a football, rather than a basketball. The eye has a short radius in one axis and a longer one in the other. This causes light to focus in two different meridians that are perpendicular to each other. Astigmatism can cause blurry vision at far and near distances. This can lead to eyestrain and headaches.
To correct for astigmatism cylindrical eyeglass lenses are required that need to be oriented properly along a certain axis. If the astigmatism power is correct in a prescription, but the axis is off vision will be blurry. This is especially true in high amounts of astigmatism. With the proper lens & orientation the focal point of both meridians will be at the same point. Contact lenses that correct for astigmatism need to be properly oriented on the eye also. This is usually done by having a weight on one portion of the lens to allow gravity to move it into the proper orientation. LASIK corrects for astigmatism by removing tissue in an elliptical fashion or more in one meridian than the other.
Presbyopia
Presbyopia is a situation where the accommodating or focusing power of the eye has diminished so much that it becomes very difficult to read material at a near distance with a full distance correction on. This is a result of the increased size and decreased flexibility of the crystalline lens. It first becomes noticeable in the forties.
Presbyopia is corrected by the use of over-the-counter readers or with bifocals. Patients with a small amount of myopia are able to read up-close because they have a “built-in” bifocal. This makes it easy for them because all they have to do to read up-close is to take their glasses off. Unfortunately, LASIK is not able to correct for this. However, some patients can overcome this by utilizing monovision where one eye is focused for distance vision and the other is focused for near vision. In order for this to work the brain has to be able to adapt to the eyes not being focused at the same point, which affects depth perception.