How to Read Your Glasses Prescription

Last Updated on April 28, 2023 by amy

Stepping out of the doctor’s examination room, you get your glasses prescription and head to the shop to finally get some new frames. But arriving at the shop, you’re disheartened that none of the employees there can make sense of your glasses prescription. What is this blasphemy? Reading a glasses prescription can’t be that hard, can it?

You snatch away the white piece of paper with a rather aggravated demeanor to take a look for yourself. Let’s see…Leslie Smith, M.D. is at the top of the paper: good to know that the professional who handed you this document is certified—at least on this piece of paper. But scrolling down the glasses prescription, you come across many incomprehensible words, letters, and numbers. Sphere? Cylinder? OD? OS? What does all this even mean?

Fortunately, this kind of scenario does not usually happen. Being a professional, your doctor will explain what everything means. But even better, the employees at a glasses shop will probably already know how to read a glasses prescription and get straight away to fit your new frames.

So maybe understanding and reading a glasses prescription is not quite a necessity to everyday living, but it doesn’t hurt to know what everything means, right? Many things to understand about a glasses prescription can be technical, but we’ll try to make this as simple as possible.

The OD and OS are probably the most straightforward elements of the glasses prescription. One of the first things you will notice on a prescription is these letters. They are usually rows located on the left-hand side of the glasses prescription, and they are simply abbreviations for the Latin terms meaning “right eye” and “left eye.” OD stands for oculus dexter, and OS for oculus sinister. On specific prescriptions is also OU which stands for oculus uterque—simply meaning “both eyes.” The OD and OS are just there to show whether the glasses prescription is for your left eye or your right eye—a pretty important component if you have a different vision in each eye.

Sphere (SPH)
One of the subsequent components you will come across on your glasses prescription is a column labeled “sphere.” While the OD and OS act only as indicators of which eye the prescription applies to, the sphere column is where the actual visual correction begins. The sphere shows the glasses’ prescription power needed to remedy vision properly.

You can understand the spherical component of the glasses prescription to be the primary element for correction. A spherical lens can magnify, blur, or correct blur. Think of a magnifying glass—a type of spherical lens that indiscriminately applies the same magnifying power over whatever object is in its field. And this is what the glasses prescription in the sphere column does: it applies a level of corrective vision over all the meridians, or points, of vision.

There are usually numbers with either a minus or a plus sign in this section of the glasses prescription. They essentially indicate whether you, as the patient, are nearsighted or farsighted. In the case that the number indicated is negative, it signifies nearsightedness. For instance, if the glasses prescription shows “-1.00”, you have one diopter of nearsightedness.

At this point, you might ask yourself, “What in the world is a diopter?” Very simply, it’s a measuring unit of the optical power of a lens. It can indicate the severity of nearsightedness or farsightedness—the stronger the lens, the greater the need for eyesight correction. So a “+4.50” on the glasses prescription would denote a higher level of farsightedness than “+1.00”.

Most people tend to have a different level of correctional need in each eye, which explains why OD and OS exist on the glasses prescription: you may have a greater level of nearsightedness in one eye than the other. OU is the only exception, which indicates that you have the same glasses prescription in both eyes. Lucky you.

Cylinder (CYL)
We’ve established that the spherical component is the primary correction in the glasses prescription. The next column on that piece of paper is the cylindrical component, like the fine-tuning of the vision. The cylinder applies a correction on top of the sphere, but in a specific direction—horizontally or vertically—as opposed to the sphere’s general, circular application.

The cylindrical component’s primary importance is its use in correcting astigmatism. It indicates whether or not you have astigmatism, the severity of it, and the lens power required to correct it. Simply put, “no astigmatism = no cylinder”—a spherical correction will be enough to correct your vision.

As in the sphere column, the cylinder column will have either a plus or minus preceding the number. And like the sphere column, the plus or minus indicate either nearsightedness or farsightedness, but this one is for astigmatism. So it’s possible to have a negative value in the sphere column and a positive value in the cylinder column.

In any case, in a glasses prescription, the cylindrical correction comes after the spherical correction; when an eye doctor examines your eye, he finds the best spherical correction and compensates for astigmatism by applying the exact amount of cylindrical correction.

Think of the axis of a car—its primary purpose is to rotate. Likewise, the purpose of the axis in a glasses prescription is to rotate. But in relation to prescription, the axis is the cylindrical lens’s orientation or rotation. As mentioned earlier, the spherical correction only applies a special power equally over all points of vision. So if you were to take a magnifying glass over a bunch of letters and rotate it, the rotation would not affect the magnification or the blur over the letters.

But unlike regular visual imperfections, astigmatism creates a directional blur. This is why a cylindrical correction is essential for anyone with astigmatism—it has a refractive power in a particular direction. Think of a bar magnifying glass: if you hold it over a bunch of letters horizontally, only their height will be enlarged, and if you hold it vertically, only their width will be enlarged—in short, its power is specific to a direction. Like the bar magnifying glass, the axis and its orientation affect what and how an object is magnified or blurred.

The axis value is measured between 0 and 180 degrees and is inherent if a cylinder value is present. So you might have something like this on your glasses prescription: “-2.00, +1.50, x180,” meaning that you have two diopters of nearsightedness, 1.5 diopters of astigmatism, and an axis of 180 degrees.

At this point begins a less common feature of the glasses prescription. The add is the magnifying power at the bottom part of multifocal lenses to correct nearsightedness.

You might’ve wondered at times why some people wearing glasses had those weird bumps or bulges at the bottom part of their lenses, and it’s precisely because of the add. So sometimes people will use glasses to correct farsightedness but also want to use the same glasses for reading. Instead of getting two different pairs of glasses which can be a bit of a hassle, they’ll get two different lenses fused into one—the upper half for seeing distant objects, the lower half for seeing near objects.

The number for the add a glasses prescription will always be a plus power regardless of whether there is a plus sign. The range is typically between 0.75 to 3.00, and unlike the other segments of the glasses prescription, the power will be the same for both eyes.

The other less common feature of the glasses prescription is the prism. Prism corrections are normally used for eye alignment problems. Sometimes an object that one eye sees is not aligned with what the other can visualize, creating the impression that there are two objects. Prism correction shifts the viewed image of one eye toward a direction so that the object is aligned correctly and the visualization is synced between both eyes.

However, not many people require a prism correction in their glasses prescription, so this column will usually be empty. Unlike a sphere, cylinder, or axis, the prism denotes prismatic power and is measured in a particular measurement called prism diopters. It is typically indicated by metric or fractional English units such as 0.5 or 1/2. So again, a prism is not especially common on a glasses prescription, but it is something that’s there if needed.

Think you can read your glasses prescription now? Perhaps the technical part can still be somewhat confusing, but knowing how to read your glasses prescription is not entirely necessary. As long as you have some sense of the significance of each section and the numbers, you’ll get a pretty good sense of where your vision stands.