Eye Glass Prescriptions – Part Two
Cylinder (CYL). This indicates the amount of lens power for astigmatism and represents the difference in the greatest power of the eye and weakest power of the eye, usually separated by 90 degrees. If nothing appears in this column, either you have no astigmatism, or your astigmatism is so slight that it is not necessary to correct it with your eyeglass lenses. 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.
Axis. If an eyeglass prescription includes cylinder power, it also must include an axis value which follows the CYL power. The axis indicates the angle (in degrees) between the two meridians of an astigmatic eye. 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.
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.
Prism. Only a small percentage of eyeglass prescriptions include prism. It is often prescribed to displace the image in a certain direction for patients with eye alignment problems such as crossed-eye (strabismus) or other eye muscle or focusing disorders.
It’s important to understand that an eye glass prescription is not the same as a contact lens prescription. An eye glass prescription is for the purchase of eye glasses only. It does not contain certain information that is crucial to a contact lens prescription and that can be obtained only during a contact lens exam and fitting. One of the reasons that eye glass and contact lens prescriptions are not the same is that eye glass lenses are positioned at a distance from the eyes, while contacts rest directly on the eyes. That distance affects the lens power required for eyes to focus properly. In addition to the information in an eyeglass prescription, a contact lens prescription must specify the base (central) curve of the back surface of the contact lens, the lens diameter and the specific manufacturer and brand name of the lens.