Contact Prescription

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Contact lens or eyeglasses prescription is a written order by an eye doctor to an optician for vision correction eyeglasses or contact lenses.

Contact lens or eyeglass prescription specifies the parameters to which the eyeglasses or contact lenses are to make in order to correct blurred vision due to refractive errors.

Eyeglasses Prescription

Eye test prescription is similar to the one in the shown diagram.

   S or Sph C or  Cyl Axis Prism VA Rdg Add Near VA
 Right Eye(OD)                         
 Left Eye (OS)              

Look at the prescription for glasses or lenses; there will be numbers listed under the headings of OS and OD. These are Latin abbreviations; OS (oculus sinister) means the left eye, and OD (oculus dextrus) means the right eye. Occasionally, there will be a notation for OU, which means something involving both eyes (right as well as left).

In general, anything that is away from zero numbers on the prescription, the worse the eyesight and more the vision correction that requires. A plus sign in front of the number means farsighted, and a minus sign means nearsighted. These numbers are representing with a unit called diopters; the unit used to measure the correction or focusing power of the lens that is required. Diopter is often abbreviating as "D."


For example, if the prescription says -1.00, means to have one diopter of nearsightedness. This is a mild amount of nearsightedness. If the prescription says -4.25, that means to have 4 and 1/4 diopters of nearsightedness. This is more nearsighted than -1.00, and requires stronger (thicker) lenses. Similarly, +1.00 would be a small amount of farsightedness, and +5 would be more.

For people who have astigmatism, there will be three numbers in the prescription. The general form for writing these numbers is S x C x Axis.

The S or sph refers to the "spherical" portion of the prescription, which is the degree of nearsightedness or farsightedness discussed above.

The C or cyl refers to the "cylinder" or astigmatism, and can be a negative or a positive number. It measures in diopters the degree of astigmatism that one have. The bigger this number, the more astigmatism one have. Astigmatism is caused when the eye is not perfectly spherical (such as a football) it shaped slightly elongated (such as a rugby ball). This causes the vision to distort for both distance and near objects. The cylinder may be plus or minus regardless of whether the sphere is positive or negative.

The "axis" represents the orientation of the cylinder in degrees (from 0-180) angle at which the lens is set into the frame. It reveals where the difference in the astigmatism occurs. It is not enough to specify how much astigmatism there is; it also has to know where the difference in curvature is taking place.

The “prism” is usually prescribing in lenses to help one to use the eyes together. Some people's eyes have a tendency to try to pull apart when they are in use; one goes up and down, and another goes in and out. These are all due to muscle imbalances or fixation. Prism is the correction needed (if any) to align the eyes, so that they are looking straight and working together. A prism in a lens will bend the path of light without altering its focus.

"VA" stands for visual acuity, indicates the standard of vision (for each eye) when corrected. It may or may not be included in the prescription and is usually presented as a fraction. In the UK, VA is measures at 6meters, so the numerator is 6 (e.g. 6/6). In the US, VA is measures at 20 feet, so the numerator is 20 (e.g. 20/20). The larger the denominator, the worse the eye sees, so a VA of 6/12 (or 20/40) is half as good as 6/6 (or 20/20).

"Rdg add" stands for reading addition, indicates the additional positive power that is required to make the eyes to focus for close work. This is usually needed for patients over 40 to 45 years, since focusing ability declines with aging/older (presbyopia). If a reading addition is stated, this means it to need different spectacle prescriptions for reading and for distance.

"Near VA" indicates the smallest sized print that can read with the prescription. It is written as Nx where x is a number indicating the print size. N5 is the smallest sized type that will normally find, and N8 is the usual size of normal newsprint.

Here are two examples of what prescriptions for eyes with astigmatism could look like.
-2.00 +1.50 x 180
+3.50 +3.00 x 45

The first prescription means that the person has 2 diopters of nearsightedness with 1.5 diopters of astigmatism and an axis of 180 degrees.

The second prescription means that the person has 3.5 diopters of farsightedness with 3 diopters of astigmatism and an axis of 45 degrees.

Contact Lens prescription

Contact lens
How do you read Contact lens prescription?

Eye Power (sphere) BC  Dia Brand
OD (Right)        
OS (Left)         

Explanation for symbols

OD – Right eye
OS – Left eye
OU – Both eyes
BC – Base curve, inside curve of the contact lens (value range from 8.0 to 9.5)
Dia – Diameter, Size of the contact lens. The range is from 13.0 to 15.0 millimeters.
Pow or Power – Spherical value, Strength of the corrective lens in diopter
for astigmatism correction
PL or Plano - No refractive error
Cyl – is Cylinder, Strength of the astigmatism.
Axis – is orientation of the astigmatism, where the difference in curvature is taking place.
Add – is bifocal plus power for near distance. This is usually needed for patients over 40 to 45 years, since focusing ability declines with aging/older (presbyopia).

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