With hyperopia, the cornea does not bend incoming light enough, so light focuses behind the retina (Fig 3). Hyperopia causes blurry vision up close and at distance. Hyperopia must be distinguished from presbyopia, which is clear vision at distance with a need for reading glasses up close.

When LASIK is performed for hyperopia,the corneal surface is gently steepened by the laser, resulting in more bending of incoming light and proper focusing on the retina (Fig 4).

With Myopia, the cornea is too steeply curved and too much focusing power is produced. This makes distant objects blurry, because light is focused in front of the retina. Near objects remain clear (Fig 1).

When LASIK is performed for myopia, the laser gently flattens the cornea, which lessens the bending of incoming light, thereby allowing it to focus on the retina (Fig 2).

Astigmatism causes light entering the eye at different axes to be focused different amounts. For example, light entering vertically (from 12 o'clock to 6 o'clock) may be focused more than light entering horizontally (from 9o'clock to 3o'clock). In an eye without astigmatism, light is focused the same amount in each axis.

The net result of astigmatism is blurred vision. Often letters appear slanted or with "tails" coming off of them. Sometimes the affected eye sees double.

When LASIK is performed for astigmatism, the cornea is reshaped by the laser to allow proper focusing, regardless of the axis. In practical terms, astigmatism is treated similar to nearsightedness and farsightedness, but with different amounts of treatment to each axis.