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Understanding Your Eyeglass Prescription

eyeglass prescription

Your eyeglass prescription is a measure of the lens strength that best corrects your vision. In your prescription a lens power is described for both of your eyes, the left and your right.

Single Vision Lens Powers

Most eyeglass prescriptions will contain two lines that describe the lens power, by convention the first line is for your right eye and the second your left. The lens power describes one of two types of lenses, a spherical lens or a cylindrical (compound) lens. A sperical lens will only have a power indicated only for the sphere (sph.) component of your lens. A cylindrical lens prescription will contain a sphere power, cylinder power, and cylinder axis. Example:

Right -3.00 sph
Left -3.00 -.50 x 180

In this example the right eye power (first line) is a -3.00 sphere. You will sometimes see sph wriiten to indicate that this lens is spherical and the cylinder was not over looked. The left eye power  is cylindrical and has a spher power of -3.00 and a cylinder power of -.50 at an axis (location) of 180.

Right eye may also be indicated by R RE OD. Left eye by L LE OS.

Pupillary Distance

Your pupillary distance is the distance between your eyes in millimeters. When eyeglasses are fabricated this measurement is used to place the optical center of your eyeglass lenses directly in front of your eyes. Pupillary distance can be further indicated by a distance and near PD. When you gaze off into the distance your eyes are slightly further apart than when you gaze at a close up object such as a book. Both eyes naturally turn in when looking at the closer object. Generally the disfference between a far PD measurement and a near PD measurement is 3 millimeters. When both the far and near PD are written you will you 65/62 for example as the PD.

Another time your PD may conatin two numbers is when the measrement is written individually for each of your eyes. You may see 33/32 for a PD. This means that from center out to the middle of your pupil for the right eye is 33 millimeters and for the left 32 millimeters. Again the right eye is always indicated first.

Addition (ADD) Power

If you require an add power in your prescription this indicates a bifocal prescription. The add power is added to the distance power to provide clear near vision for those requiring an add. Power of the addition generally ranges in the +.75 to +3.00 range. Example:

OD +1.25 -1.50 x 90
OS +1.00 -1.75 x 95
Add +2.25

It is very important to note the lens power my be either + (plus) or - (minus). A +1.00 lens is completely differnt tha a -1.00 lens.

By Dr. William Becker