Sometimes, for different reasons, the eye doesn't focus quite right:
- The surfaces of the lens or cornea may not be smooth, causing an aberration that results in a streak of distortion called astigmatism.
- The lens may not be able to change its curve to properly match the image (called accommodation).
- The cornea may not be shaped properly, resulting in blurred vision.
Most vision problems occur when the eye cannot focus the image onto the retina. Here are a few of the most common problems:
- Myopia (nearsightedness) occurs when a distant object looks blurred because the image comes into focus before it reaches the retina. Myopia can be corrected with a minus lens, which moves the focus farther back.
- Hyperopia (farsightedness) occurs when a close object looks blurred because the image doesn't come into focus before it gets to the retina. Hyperopia, which can also occur as we age, can be corrected with a plus lens. Bifocal lenses, which have a small plus segment, can help a farsighted person read or do close work, such as sewing.
- Astigmatism is caused by a distortion that results in a second focal point. It can be corrected with a cylinder curve.
In addition, lenses can be made to correct for double vision when the eyes do not work together ("crossed eyes"). The lenses do this by moving the image to match the wayward eye.
Corrective lenses, then, are prescribed to correct for aberrations, to adjust the focal point onto the retina or to compensate for other abnormalities. You can read more about vision problems in How Refractive Vision Problems Work.