Majority of the image focusing is done by the cornea. The crystalline lens inside provides the fine focusing power. These two, work together and focus rays of light on the retina (the screen in the eye) to form an image.
In a normal eye, the cornea and the lens focus light rays entering the eye on to the retina to produce clear vision. Hereditary or developmental factors however, create the following focusing problems.
a) MYOPIA (Short sight)
Usually inherited; the myopic eye is longer than the normal eye wherein the light rays focus in front of and not on to the retina. This leads to a blurred vision. Increase in height is generally associated with increase in spectacle number, which usually stabilises by the age of 18 years.
b) HYPERMETROPIA (Long sight) –
Hypermetropic people have a focusing problem caused by an eyeball which is shorter than the normal eye. Thus light rays focus behind the retina resulting in unclear vision. The effects of long sightedness vary with age, because of the diminishing flexibility of lens inside the eye.
Here the eye is unable to focus clearly in all the meridians. Astigmatic corneas, instead of being spherical, have more curvature in one meridian than the other. Astigmatism often occurs in conjunction with short sight or long sight.