People with vision problems must have their eyes tested regularly to get glasses. In this testing, the patient looks through a machine called a photoroptor. This machine shines a light into each eye to determine the amount of refractive error in the eye's structure. (Reference 1) The lenses of glasses will be shaped to correct this vision error. The shape of the lenses necessary is written out as a prescription to be given to an optician. Understanding the meaning of these numbers allows you to understand the prescription.
Look at your prescription. A typical one might look like this:
OD: -5.00 +2.00 x 90
OS: -6.00 +2.00 x 95
Read the first number of the first row. "OD" stands for "oculus dexter," which means "right eye" in Latin. The first number (with a plus or minus sign in front of it) is the sphere measurement indicating nearsighted or farsighted measurement for the right eye. A minus sign indicates myopia, otherwise known as nearsightedness, while a "plus" sign indicates hyperopia, or farsightedness.
Understand the first number. It is a measure of how strong the vision problem is, in dioptres. A dioptre describes the distance from the eye at which it can clearly focus. For example, at minus-1 dioptre this distance is 1 meter, while at minus-10, it is 1/10th of a meter. For nearsighted eyes, dioptre measurements of minus-3 or above are considered "mild," while those between minus-3 and minus-6 are classed as "moderate." Anything beyond minus-6 is considered "severe." For farsighted eyes, the scale is the same in the positive range; a dioptre of 0 to 3 is mild, 3 to 6 is moderate, and anything above 6 is severe.
Move to the right in the first row on your prescription. Any other numbers here are to measure the amount and direction of astigmatism (blurriness from corneal problems) in that eye. The first number is called the "cylinder" and measures the strength of the astigmatism, again in dioptres. The second number measures what direction the spot is in, plotted in degrees, like on a protractor. (Reference 2)
Read the second row of your prescription. It starts with "OS," which stands for "oculus sinister," which in Latin means "left eye." The numbers in this row describe the sphere, axis and cylinder for the left lens, which may be quite different than the measurements for the right.
Look beneath the "OS" and "OD" rows. If your prescription says "Add:" and then provides a number, than it is a prescription for bifocal glasses. The "Add" is the strength, in dioptres, of the near focusing area of a bifocal lens. Some prescriptions may have two numbers here, indicating that the lens is trifocal. In such glasses, the wearer can look through three areas with different optical properties to help with different vision problems.