Understanding your soft contact lens prescription and terminology

One of the most common forms of corrective lens is the soft contact lens. Your prescription for soft contacts will include a particular brand.Each brand has different styles, materials and purposes. Not all brands require the same measurements such as the curvature and diameter of the lens. Your prescription will also include:


Each brand of lens is used to treat different kinds of refractory errors. Your doctor will prescribe the best brand to meet your needs.

Right Eye (OD)

The Latin phrase for "right eye" is oculus dexter (OD). If your prescription is the same in both the left and right eye, "ocular uniter" (OU), meaning both eyes, is used instead.

Left Eye (OS)

The Latin phrase for "left eye" is oculus sinister (OS). If your prescription is the same in both the left and right eye, "ocular uniter" (OU), meaning both eyes, is used instead.


Power or sphere, of a soft contact lens is calculated in a unit of measurement equal to the reciprocal of the focal length of a lens called a diopter. The spherical power of a lens is indicated with a plus sign (+) for hyperopia and a minus sign (-) for myopia.

Base Curve

The base curve (BC) indicates the curvature on the inside of a lens. This measurement is noted in millimeters, usually between 8 and 10, and attempts to come as close to the curve of the eye as possible to ensure the greatest fit and comfort. If the cornea is more bulged or curved, this number will be lower.


Abbreviated "dia," the diameter of a lens measures the width of a lens in millimeters, from end to end. The average size of a contact lens is between 13.5 and 15 millimeters.


Although color is generally not used in the treatment of a refractory error, lenses do come in numerous colors or tints. Visibility tints don't change the way a lens functions but are intended to help the wearer differentiate lenses from a storage case. Enhancers are tinted lenses designed to enhance or embellish an eye's natural color. Opaque contacts are designed to mask the eye color of someone with dark eyes in a way that keeps the original iris color from showing through. Opaque contacts and enhancers are quite popular but are generally noticeable when compared to natural eye colors.

Add Power

For individuals with presbyopia alone or in conjunction with another refractory error, bifocal or multifocal lenses are often needed. In this case, an "add" measurement is also used in a prescription. This measurement indicates the additional amount of power needed to ensure clear vision at a close range and is always preceded by a plus (+) sign.


The cylinder power of a lens measures the degree to which a lens must account for astigmatism. The cylinder is an oval band on a lens that enables an individual to see all ranges clearly at once and is measured in diopters, like spherical power. The cylinder power of a lens is typically expressed with a minus (-) sign and is always written separately from the sphere power to avoid confusion.


The axis indicates the orientation of the astigmatism. This measurement designates where the cylinder correction should be located. Generally, the cylinder runs up and down or side to side.The measurement will be between 90 and 180 degrees and will be proceeded by the symbol "x".