How to Read Your Glasses Prescription
When you finish your eye exam each year, the doctor will hand you a prescription containing a cryptic collection of numbers, plus and minus signs, abbreviations and words. If you’re like most people, this cluster of numeric information is fairly indecipherable.
However, these numbers and letters describe your ocular health — and figuring out what they mean isn’t as hard as it seems. This article will explain the specifications of your glasses prescription, which will enable you to be more involved and have a better understanding of how optometrists measure your vision.
Many people wonder why glasses prescriptions have to be so complicated. The answer is that they are actually miraculously simple — four to six columns and two to three rows can describe the entire scope of your vision. If you’ve ever studied the human eye, you know what a dazzlingly complex and multi-faceted organ it is. The fact that a single chart can compile its abilities into a usable format is a spectacular feat of science and medicine.
Let’s take a look at what vision conditions can be treated with glasses, how glasses work and the meanings of the different numbers, abbreviations and words in a prescription.
Before we begin, it will help to be familiar with the following eye terminology:
- Cornea: This is the transparent lens through which light passes into your eye. Like the lens of a camera or magnifying glass, it takes incoming light and focuses it through the eyeball and onto the retina. The analogy of the cornea is a movie projector — it shines light through a lens and focuses it according to the distance of the screen.
- Retina: The retina is a light-sensitive layer of cells on the back of the eyeball. Photons of light land on this layer and trigger the underlying nerves, which transmit signals to the brain that form an image. This part of the eye is analogous to a movie screen, upon which the projector will shine the light.
What Vision Condition Do You Have?
Corrective eyewear is prescribed to treat a number of common vision conditions. Start by identifying which of the following conditions, if any, apply to you:
- Astigmatism: This condition describes the irregular shape of the eye’s cornea. This asymmetry causes vision to be blurred or distorted. Those affected by it squint to compensate, which leads to symptoms like muscle fatigue and headaches. Astigmatism is likely hereditary and is present in most people. It is often left untreated in less serious cases, though glasses can make an enormous difference in the quality of life by correcting the problem.
- Hyperopia: Otherwise known as farsightedness, this condition involves functional vision at a good distance but blurry, unfocused vision at close range. The cause of this is an eyeball that is too short between its cornea and retina, or a cornea with insufficient curvature to properly focus light onto the retina. Much like a projector that has not been correctly focused, this causes light to converge beyond the retina. Mild farsightedness may be tolerable without glasses, but the problem is easily correctable.
- Myopia: Otherwise known as nearsightedness, myopia is the inverse of hyperopia — its symptoms include detail and focus at close range, while objects at a distance remain fuzzy and unclear. It is typically a result of too great a distance between cornea and retina. This means the light is focusing within the interior of the eye instead of on the retina itself.
- Presbyopia: As eyes age, the cornea usually becomes less flexible. Because this flexibility is crucial to focusing on details like written words, presbyopia results in a loss of ability to read or see things at close distances, while objects far away are still clearly visible. While this is mostly an unavoidable part of aging, glasses can quickly and easily correct it.
How Do Glasses Work?
The concept of glasses has been around since the Roman tragedian Seneca claimed to have read books using a glass bowl of water as a magnifying glass. The first modern glasses, however, did not likely appear until late in the 13th century in northern Italy. The first known painting of glasses was done in 1352 and depicted a monk with a pair on his nose.
At any rate, the physical principles behind how glasses work deal with how light bends. The vision disorders listed in the last section concern the focal length of the eye and the shape and flexibility of the cornea. In all of these conditions, the issue is that incoming light is not being correctly focused on the retina. This is why images look blurry — the light needs either a sharper or gentler angle to get in focus.
When light passes from one medium to another, whether it is clear glass or a cornea, it refracts a certain amount. The exact amount it bends is reliant upon physical properties of the medium, but it is also proportional to the surface’s angle to the perpendicular. In other words, light entering at the center of the cornea bends less than light entering at the edges — because the edges of the cornea are at an angle greater than 90º to the incoming light rays.
When the cornea does not bend light sharply enough to focus it on the retina, the lens of prescription glasses compensates by bending it an extra amount. So by the time the light passes through the lens of your glasses, it has already been focused by the same amount that your cornea lacks. Hence, the combination of lens and cornea cause it to focus perfectly on your retina.
What Do OD and OS Mean?
Let’s now look at the left side of any glasses prescription. There will be two rows entitled “OD” and “OS,” which translate to the following:
- OD: This stands for “oculus dexter,” or “right eye” in Latin.
- OS: This stands for “oculus sinister,” or “left eye” in Latin.
You might also see glasses prescriptions that include “OU.” This stands for “oculus uterque,” or “both eyes” in Latin.
What this means is that the rows of your eye exam are broken into two separate rows — one for your right eye and another for your left eye. Occasionally, another row will describe qualities of both eyes.
What Are SPH and CYL?
Two of the columns on your glasses prescription will be dedicated to the headings SPH and CYL, which stand for “sphere” and “cylinder.”
Sphere describes the amount of correction required to treat either farsightedness or nearsightedness. In other words, it describes the power of a lens. Positive numbers describe a convergent, convex lens, while negative numbers describe a divergent lens. A convex lens is needed to treat hyperopia or presbyopia, while a concave lens treats myopia by spreading light out.
Cylinder provides an indication of astigmatism and the strength of lens needed to correct it. In this condition, the shape of the cornea is distorted and causes vision to be similar distorted or blurry. Hence, a spherical lens is not sufficient to correct for astigmatism. Rather, astigmatism requires use of a toric lens — that is, a lens with focusing power along two perpendicular axes — to compensate for irregularly shaped corneas.
What Do Axis, Add and Prism Mean?
Other columns on a glasses prescription include “Axis,” “Add” and “Prism.” These delineate further characteristics of a glasses lens:
- Axis: As you might have gathered in the previous section, correcting for astigmatism requires a somewhat complex series of adjustments. Simply put, Axis is a measurement of the orientation and degree of astigmatism. Note that if a lens is purely spherical — that is, if there is no CYL component on a glasses description — then Axis is not needed.
- Add: This column describes the power of magnification in the bottom portion of a lens, most useful in treating presbyopia. Lenses can have different parts for viewing different distances. The lower portion of a multi-focal lens is typically reserved for magnification. This allows those who struggle to see up close, most often a problem when reading or writing, to simply glance down through the magnifying portion of the lens and see clearly.
- Prism: A small percentage of prescriptions contain Prism, which is a measurement of prism diopters in a lens. These serve to correct problems with eye alignment. The orientation of the prism is described by abbreviations for base up, base down, base in or base out — BU, BD, BI or BO, respectively. This number typically comes in decimal form.
Is This the Same as a Contact Prescription?
Glasses prescriptions are not the same as contact lens prescriptions and hence are not interchangeable. They do share some similarities, but they operate in different ways.
A prescription for contact lenses denotes the power needed to correct for refraction errors. These may be due to myopia, hyperopia or astigmatism. However, the contact lens is sitting directly on your cornea — not on the bridge of your nose, a full centimeter away from your eye. For this reason, the refractive power of contact lenses is bound to be quite different from that in your glasses prescription. This allows the lens to focus light in the most beneficial way.
In addition, a prescription for contact lenses includes a different set of variables than a glasses prescription. Some examples of these are:
- Diameter: Contact lenses have a specific diameter that corresponds to the size of your cornea. This affects the lens’ fit on your eye. Soft contact lenses tend to have a different diameter than rigid gas permeable lenses.
- Expiration: Prescription contact lenses generally last for a year, after which another doctor’s appointment is required.
- Base Curve: The back surface of the contact lens must adhere to your cornea, and so it has its own specification to ensure a comfortable and effective fit.
- Lens Material: The material a contact lens is made of allows for certain degrees of oxygen permeability — that is, how much oxygen can get through the lens to nourish the cornea. This is an important consideration for those hoping to use extended wear contact lenses.
It is sometimes the case that a person will wear glasses and contact lenses in conjunction. This is an effective means of correcting vision without the use of overly thick glasses or contacts alone. Additionally, contact lenses may be used for style, as they can provide different eye colors.
Can You Keep the Prescription to Use Later?
This largely depends on how you define “later.” If you plan on getting your glasses in a few weeks or a month, your prescription will still be valid. However, your eyeglass prescription will generally expire after a year or two, and you should not wait this long to fill it. At the one year mark, most glasses prescriptions expire. You should have an eye appointment each year to update your prescription.
This is the case even if you feel your vision is fine with your older prescription. Our eyes naturally change, and in doing so adjust to glasses. The result is that your eyes may be straining to clarify vision through your current prescription. One sign of this is the presence of headaches, neck soreness or eye fatigue. Just assume that you will be in need of a new prescription every year or two, or as recommended by your doctor.
The Importance of Updating Your Prescription
Beyond eye strain, there are other reasons you should be updating your prescription. Optometrists, researchers and eyewear manufacturers are constantly developing new products, and by updating your prescription, you are taking advantage of the newest technology. Updated technology means clearer vision, lighter materials, fewer headaches and stress points, better durability and the newest styles.
Optometrists also know what to look for in terms of replacements. Whether your glasses have barely visible scuffs, scratches or damage to moving parts, replacing your glasses or lenses will help ensure you don’t have to struggle with damage throughout the year.
The Advantages of Kounopt Glasses
At Kounopt, we have over 30 years of experience in leading the industry’s drive for better technology, maximum comfort and effortless vision. Using multi-million-dollar equipment and some of the most cutting-edge minds in optics, we take every step to ensure the widest field of view and the most advanced technology.
When you purchase your eyewear from Kounopt, you are taking advantage of the following benefits:
- Speed: We understand the excitement of ordering a new pair of glasses — no other product has quite so profound an impact on our look and style, as well as on our comfort. We can ship completed eyeglasses in less than 24 hours, or even faster in the case of an emergency.
- An Expert Team: Creating great glasses is both an art and a science, but science undoubtedly plays the biggest role. That’s why our team includes optometrists and opticians — our products come from qualified experts who understand eyewear and patients alike.
- Top-of-the-Line Machinery: Creating consistently high-quality products requires the right tools. At Kounopt, we use the latest hi-tech, freeform machinery.
- Authenticity: We always ensure authentic brands and lenses. That means no copies or knock-offs.
- Satisfaction Guarantee: We back our products with a 100% money back guarantee. If you don’t like it, you don’t pay for it.
- Free Shipping Worldwide: For orders over $30, we offer free shipping worldwide. Combined with our speedy delivery methods, this means a satisfactory delivery process.
- Price Match Guarantee: Visit our website to read terms and conditions for our price match guarantee.
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