Lens system

Lens system of the eye consists of a convex lens and the lens placed in front of her and curved fluid-filled membrane called the cornea. The cornea provides four-fifths of the whole process of focusing. Fine adjustment is performed by the crystalline lens, whose curvature of the surface changes surrounding muscular ring (capsule). When the eye is unable to obtain the necessary forms, usually because of abnormalities in these muscles, images of visible objects become indistinct.
The most common lack of vision inability to focus on retina image of individual objects. If the lens system of the eye is too strong, in other words, if it is very convex, distant objects will be blowouts, and close to give a clear image. People with such disorders are called nearsighted. If the convex lens is insufficient, rasplyvetsya close objects, but will remain a clear image of distant. Those who have such vision is called farsightedness. Both disorders can be corrected by using glasses or contact lenses.
Nearsighted people wear glasses with concave lens (thinner in the middle), which allows their eyes to focus on distant objects. Farsightedness using glasses with convex lenses (thicker in the center).