Common Refractive Problems
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:
- An eyeball that is too long
- A cornea that has too much curvature
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:
- An eyeball that is too short
- A cornea that has too little curvature
Here the image falls behind the retina. Converging lenses are used to correct this defect.
By age forty, many peoples 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.
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 it.
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.