Understanding Your Prescription
Your prescription describes the degree your eyes are out of focus and specifies the lens strength needed to correct your vision. An optician will use your prescription to provide you with the appropriate strength of glasses or contacts.
Contact Lens Prescription
Your contact lens prescription differs from an eyeglass prescription and can only be obtained during a contact lens consultation and fitting.
In addition to the information in your eyeglass prescription, your contact lens prescription also contains:
- Information specifying the central curve of the contact lens' back surface
- The contact lens diameter
- The specific manufacturer and brand name of the contact lens
As contact lenses rest directly on your eye, the strength of your contact lens prescription can and often does slightly differ from your eyeglass prescription.
Sphere (SPH). This indicates the amount of lens power, measured in diopters (D), prescribed to correct nearsightedness or farsightedness. If the number appearing under this heading has a minus sign (–), you are nearsighted; if the number has a plus sign (+) or is not preceded by a plus sign or a minus sign, you are farsighted.
The term "sphere" means that the correction for nearsightedness or farsightedness is "spherical," or equal in all meridians of the eye.
Cylinder (CYL). This indicates the amount of lens power for astigmatism. If nothing appears in this column, either you have no astigmatism, or your astigmatism is so slight that it is not really necessary to correct it with your eyeglass lenses.
Meridians of the eye are determined by superimposing a protractor scale on the eye's front surface. The 90-degree meridian is the vertical meridian of the eye, and the 180-degree meridian is the horizontal meridian.
The term "cylinder" means that this lens power added to correct astigmatism is not spherical, but instead is shaped so one meridian has no added curvature, and the meridian perpendicular to this "no added power" meridian contains the maximum power and lens curvature to correct astigmatism.
The number in the cylinder column may be preceded with a minus sign (for the correction of nearsighted astigmatism) or a plus sign (for farsighted astigmatism). Cylinder power always follows sphere power in an eyeglass prescription.
This describes the lens meridian that contains no cylinder power to correct astigmatism. The axis is defined with a number from 1 to 180. The number 90 corresponds to the vertical meridian of the eye, and the number 180 corresponds to the horizontal meridian.
If an eyeglass prescription includes cylinder power, it also must include an axis value, which follows the cyl power and is preceded by an "x" when written freehand.
The axis is the lens meridian that is 90 degrees away from the meridian that contains the cylinder power.
Add. This is the added magnifying power applied to the bottom part of multifocal lenses to correct presbyopia. The number appearing in this section of the prescription is always a "plus" power, even if it is not preceded by a plus sign. Generally it will range from +0.75 to +3.00 D and will be the same power for both eyes.