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Understanding glasses prescription:

Don’t be daunted if you are not an eye care professional. Understanding glasses prescription is not that difficult. Just read this brief introduction and you are well on your way. But first, some common lingo:

OD means right eyes
OS means left eye
Sph is shorthand for “spherical correction.” If it is a positive number, the glasses corrects for far-sightedness
(hyperopia). If it is negative, it corrects near-sightedness (also known as myopia). People who are far-sighted may need to strain to get images into focus. Near-sighted people can see up close but not far away.

Cyl is shorthand for “cylinder,” also known as astigmatism. The higher the number, the greater the amount of astigmatism. If it is not written (or zero), it means there is no astigmatism. Astigmatism means the optical system is not perfectly spherical (like a basketball) but more like an ellipse (like rugby or American football).

Axis represents the angle, or direction, of the astigmatism. It is usually between 0 and 180 degrees.
Diopter is the unit of measurement of myopia, hyperopia, or astigmatism.
For example, the following prescription:

OD: -2.00 + 1.00 x 90
OS: +0.25 + 0.50 x 180

means the right eye has 2 diopters of near-sightedness (myopia) and 1 diopter of positive astigmatism (see below) at 90 degrees (vertical), while the left eye has 0.25 diopter of far-sightedness (hyperopia) and 0.5 diopter of positive astigmatism at 180 degrees (horizontal).

The + orsigns between the spherical and astigmatic corrections are critical because they denote different conventions in writing a prescription. The so-called “plus” and “minus” astigmatism/cylinder can be converted from one form to another, which the computer will do automatically for you. When entering a prescription, it is important to take note of this sign and choose the correct one. See a sample prescription form here.

Bifocal or Trifocals (sometimes known as “reading addition”) usually have additional segment(s) on the lens that brings closer objects into focus. You may see something like Add 2.00 or +2.00 after the usual prescription. For bifocals or progressive lenses (the additional segments on progressive lenses have a continuously increasing power from top to bottom) have only one Add, while trifocals have two Add. Usually the left and right lenses have the same Add. For example, the following prescription OD: +1.00 – 0.5 x 46 OS: +0.75 – 1.00 x 128 Add 2.50 means the right eye has 1 diopter of hyperopia and 0.5 diopter of negative astigmatism, while the left eye has 0.75 diopter of hyperopia and 1 diopter of negative astigmatism. Further more, there are 2.50 diopters of additional power in the bifocal segment in each eye that helps in reading or seeing up close.

Rarely, there are Prisms on lenses to correct double vision due to misalignment of the eye (“cross-eyed”). There are four types of prisms: base in (BI), base out (BO), base up (BU), and base down (BD). Each eye may have one or two prisms. If you are not sure about this, please consult an eye care professional. There are many other attributes to a pair of glasses, such as the lens material, the type of frame, whether it is scratch-resistant, color tinting, etc. These are all optional information to include when registering the glasses, if it is known. Of course, the more information we have, the better.