Contact lens prescription questions

Wondering what all those letters and numbers on your contact lens prescription actually mean? Here’s how they break down:

OS: Oculus Sinister – This is Latin for “Left Eye.” All the numbers pertaining to “OS” are specifically for your left eye.
OD: Oculus Dexter – This is Latin for “Right Eye.” All the numbers pertaining to “OD” are specifically for your left eye.
PWR: Refractive Power – This is the adjustment in diopters that your eye will need to achieve 20/20 vision. If the number is a negative, that means myopia (nearsightedness), if the number is a positive, this indicates hyperopia (farsightedness).
BC: Base Curve – The curvature of your cornea in millimeters
DIA: Diameter of the lens, from edge to edge
CYL: Cylinder – This number indicates correction for astigmatism, if present. If the number is a negative, that means myopic astigmatism, and if the cylinder number on the prescription is a positive, this indicates hyperopic astigmatism (note: contact lenses are dispensed with negative cylinders for hyperopic astigmatism – strange, but true!) .
AXIS: Also for astigmatism. This indicates the degree of axis of the cylinder in the lens, used to correct the effects of the cornea’s oval shape.
ADD: Add Power – Used for bifocal contact lenses.
COLOR: This is the area of the prescription where any color or style of the lenses is written.
BRAND: Here, the optician will indicate the suggested brand of lens best for you.

A contact lens fitting is a special procedure your ECP will perform in order to determine which contact lenses will work best for you. After performing an eye exam, your ECP will take the shape and size of your eyes into consideration when fitting you with lenses, as well as other factors (such as if you have dry eyes) to make sure you get a pair of lenses that fits comfortably.

Not legally. Since contact lenses are considered medical devices under the FDA, you’re required to have a valid, current prescription to buy contacts online or from your preferred retailer — even if you’re purely looking for cosmetic, non-corrective colored lenses. Nonprescription cosmetic lenses (those obtained without the advice of an eye doctor) have been reported to cause corneal damage, scarring and even vision loss.

No. Because the corrective lenses are closer to your eyes than glasses — they’re actually on them — the prescription will need to be modified before contact lenses can be dispensed. Also, contacts require a measurement of the curvature of your eye, so the lenses will fit properly.