- RX FAQs
What is "S”/SPH/Sphere?
It refers to the amount of nearsightedness (myopia) or farsightedness (hyperopia). If the sphere number is negative, the patient is nearsighted. If it is positive, he or she is farsighted. Sphere numbers of zero are called "plano" or abbreviated "PL".
What is "C"/CYL/Cylinder?
It refers to astigmatism, and can be a negative or a positive number. It measures, in diopters, the degree of astigmatism that you have. The bigger the number, the more astigmatism you have.
What is the Axis?
It is a number anywhere between 0 and 180 degrees. It reveals the orientation of the astigmatism. It is not enough to specify how much astigmatism there is; you have to know where the difference in curvature is taking place.
What is Add Power/ADD/Near?
It refers to reading glasses power and is always in increments of .25. All frames can accommodate all reading powers. Normally reading powers will be the same for the right and the left eyes.
Note: If your prescription (Rx) has a reading power, you are not required to include the reading power in your lenses. You can create single vision lenses without the reading power.
What is Pupillary Distance (PD)?
is the distance between the centers of the pupils in each eye. This measurement is used when preparing to make prescription eyeglasses.
Can I measure my own PD?
Yes, you can. It is very simple, all you need is a marker or permanent marker. Look in the mirror with your regular glasses on. Dot each of the lenses with your marker where the center of your pupils are. Measure the distance between the two dots on your lenses in millimeters. Do this by laying the ruler flat on a table, then position the left dot directly over "0" millimeters. Then look directly over the right dot, and note the millimeter number. This number will be between 55 and 70 millimeters. You can also have someone place a ruler up to each pupil and measure the distance between the pupils. See the picture below.
PD is REQUIRED FOR EVERY ORDER. If it is not on your prescription, please call your optometrist or whomever made your glasses last to get this number.
TIP: Tell your optometrist you are making safety glasses for work!