The human eye gives us the sense of sight, allowing us to learn more about the surrounding world than any of the other five senses. The eye allows us to see and interpret the shapes, colors and dimensions of objects by processing the light they reflect or give off.
The cornea (the clear window on the front of the eye) and the lens of the eye (the transparent structure inside the eye) are both critical to normal vision. The goal of these two lenses is to focus light onto a layer on the back of the eye known as the retina. As light enters into the eye it is focused by the cornea and the lens so that images appear clearly on the retina. The retina then transmits these images to the brain where they are processed. If the images focus perfectly on the retina, this result in 20/20 vision; focusing in front or behind the retina results in nearsightedness or farsightedness. If the cornea is shaped like a football instead of a sphere, this is called astigmatism. When any of these conditions occurs, images are perceived by the brain as being blurry. This is due to "refractive error" which means the eyeball is not the right size or the cornea does not have the right curve. Presbyopia is a vision condition in which the lens loses its flexibility, making it difficult to focus on close objects.
The following are common vision defects can be rectified using glasses. They are all caused by the shape and positioning of certain parts of the eye.
This is also called short-sightedness. If you have this problem, objects close at hand can be seen clearly. Objects in the distance appear blurred. This can be caused by:
The image falls before, rather than on, the retina. This is corrected by using diverging (concave) lenses.
This is called long-sightedness. In case you have this complaint you will be able to see things far away clearly and the objects close by will appear blurred. This can be caused by:
Here the image falls behind the retina. Converging lenses are used to correct this defect.
Astigmatism is caused by the irregular shape of the cornea.This causes images to be blurred.This often occurs with myopia or hypermetropia.There seems to be no specific reason for the occurrence of astigmatism.Some are born with it and live with
The corrective lenses needed when astigmatism is present are called Toric lenses and have an additional power element called a cylinder. They have greater light-bending power in one axis than in others.
By age forty, many people’s vision suffers presbyopia. The crystalline lens ability to focus on close objects or small prints gradually declines with age. Presbyopia occurs independently of other vision defects like myopia or hypermetropia.
Usually, bifocals will be prescribed for presbyopia. Here, the lower part of your lens have the power required by you to read. The upper portion of the lens enables you to see distant objects. While bifocals solve the near vision problem, there is a blurring of objects in the intermediate (neither far nor near) zone. The spectacles are thick and the double power tends to make one look older.
Lately, progressive lenses are being prescribed for presbyopia. These lenses are also called the no line bifocals. A progressive lens essentially has three zones: one for distance vision, one for intermediate vision and one for near vision. It provides a smooth transition of sight from distant objects through intermediate objects to near objects, with all the in-between corrections included as well. Progressives lenses needed spectacle frames that were quite big. But now compact progressives are also available.