If you’ve just had your first pair of glasses prescribed, you probably have lots of questions. There are so many choices out there that the selection can be overwhelming. This guide will lead you step by step though the process, helping you make sure you know enough to make the best choice in glasses for your lifestyle.
A pair of glasses is a lot like a pair of shoes; they feel great in the store, but after wearing them for a few hours, the problems show up. They may be too tight and pinch, or rub you in the wrong spots. They may be too heavy, causing them to slide down your nose. Pretty soon your glasses end up tossed in a drawer, and you’re stuck with a headache as you squint and strain. Choosing the right frame and lens is essential to the comfort and successful wear of your glasses. Read on to learn how to make the best choices for your prescription, fashion sense and lifestyle.
The Prescription
The prescription that your optometrist or ophthalmologist hands to you written on a piece of paper tells an optician the exact strength of the glasses you will need. There are four basic measurements: the sphere power, the cylindrical power, the axis, and the PD, or pupillary distance. If you need bifocals, there will also be an additional power written for those. All the powers in a pair of glasses are expressed in diopters. So what does all that mean, exactly?
A typical prescription looks like this:
OD -1.25 -1.00 x 90
OS -2.00 -1.25 x 94
+2.00 ADD PD 64/61
OD is a Latin abbreviation meaning oculus dexter, or right eye. OS, oculus sinister, signifies the left eye. The prescription for the right eye is always written first. In the above example, the prescription signifies that the person is near-sighted with a fair amount of astigmatism, and needs bifocals—glasses with an additional prescription for reading.
The first number written, -1.25, is the sphere power. This sphere power can be written for near or far-sighted powers, and measures how much near or distance power a person needs to see clearly.
The second and third numbers, -1.00 x 90, measure astigmatism, and are read in combination. The second number, called the cylinder, measures how many diopters of astigmatism a person has, and the third number is the axis. The axis tells the optician where in the lens to place the cylindrical power. The axis is measured in degrees, and can be any number between 1 and 180. Astigmatism means that the eye is not perfectly rounded, but shaped more like a football, and causes blurred vision. Most people have both sphere and astigmatism correction.
The next number, +2.00 Add, signifies that the person also needs correction to see close up, or needs reading glasses. The Add power is almost always the same in each eye. Everyone will eventually need glasses for reading or close work. As the eye ages, the elasticity of the eye’s lens decreases, making it harder and harder to focus on near objects. The technical term is presbyopia, also called “long-arm sight,” because a presbyope will start holding reading materials out at arm’s length in order to see them. Presbyopia is usually noticed around 40 years of age, and it happens to everyone.
Finally, the PD, or Pupillary Distance, is the distance from the center of the right pupil to the center of the left. Often, this is measured when you are fit for glasses by the optician, rather than by the doctor. The PD measurement is taken to ensure your lenses are centered directly in front of your line-of-sight. You will see most clearly through the direct center of the lens.