Vision Defects

The most common eye defects in humans are nearsightedness (myopia), farsightedness (hypermetropia) and astigmatism. The shape of a normal eye is such that the retina is positioned at the proper distance behind the lens and light rays converge at the fovea. In myopia, the eyeball is too elongated and the retina is too far away from the lens, so that the rays of light converge at a point in front of the retina, and at the level of the retina they diverge again, creating a blurry image.

With farsightedness, the eyeball is too short and the retina is too close to the lens; therefore, light rays hit the retina before they come into focus, which again leads to blurred images. Concave lenses correct myopia by moving the intersection of the rays back, while convex lenses correct farsightedness by causing the light rays to converge closer to the lens.

Astigmatism is when the curvature of the cornea is not the same in different planes, and therefore light rays lying in one plane are focused at a different point than rays lying in another plane. Lenses to correct astigmatism must be ground unevenly to compensate for the uneven curvature of the cornea.

In older people, the lens may lose its transparency; becoming opaque, it will block the passage of light to the retina, causing blindness. The only possible treatment is surgical removal of the lens. In this case, vision is restored, but the eye loses the ability to focus, so the operated person must wear special glasses to replace the lens.