by Tej Bhulai

Created on: January 22, 2010

How understanding your prescription can save you hundreds on eyeglass purchase.

If  you have a basic understanding of your prescription you will be able to shop more wisely and save hundreds of dollars  in the process. Myopia or Nearsightedness are people who have problems focusing on distant objects. Opticians use what they call a minus lens to correct myopia. Minus lens are characterized by being thick on the edge of the lens and thin on the center.

Hyperopia or farsightedness are those who have problems focusing on close objects. Opticians use a plus lens to correct farsightedness.  Plus lens are thick in the center and thin on the edge. Presbyopia or Greek for "elderly vision" usually occurs after forty, as the muscles of the eye loses its power to accommodate objects at near distances. Presbyopic patients are those who need correction for both distance and near. Bifocal lenses are used to correct this type of eye condition. Prescriptions are usually written in this format:

RX Spherical Cylindrical Axis

O.D. -3.25 -.25 130

O.S. +.50 -1.00 80

O.D. +2.00 ADD

O.S. +2.00 ADD

The above prescription is for someone who is Presbyopic.

O.D. or Oculus Dexter is Latin for right eye and O.S. or Oculus Sinister is Latin for left eye.  D.V. is distance vision and N.V. is near vision.  Add refers to the correction needed for reading. The first set of numbers or the spherical numbers represent the correction needed for distance, or the power the lens have to be, in order for you to see objects in the distance correctly.

The second part or the cylindrical value corrects for astigmatism. Astigmatism occurs when the cornea is shaped more like a football instead of a basketball. Because the eye rotates on a 180 degree axis, the axis part of the prescription represents the meridian where the correction is needed.

Eyeglass lenses are made of one of three materials; they are all made of plastic with what they call different indexes. Basically, the higher the index, the thinner the material is. CR39 or basic plastic is 1.498, poly is 1.586, and high-index comes in a few different flavors, with index ratios between 1.53 and 1.74. The higher the index, the thinner, lighter, and more expensive the lenses are.

Polycarbonate is good for safety, while high-index is recommended for someone with a high prescription, such as -5.00 or above. If you have a really thick prescription, polycarbonate or high index is thinner and is the recommended material for you.

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