Glasses vs. Contact Lenses Prescriptions

May 8

When it comes to vision correction, understanding the differences between contact lenses and glasses prescriptions is essential for ensuring optimal visual clarity and comfort. While both serve the same purpose of correcting refractive errors, such as myopia (nearsightedness), hyperopia (farsightedness), and astigmatism, the way prescriptions are written and interpreted varies. It’s important to notice that these prescription types include different measurements and cannot be used interchangeably.

Glasses Prescriptions

Glasses prescriptions are written for lenses placed in frames and worn in front of the eyes. They typically include the following components:

Sphere (SPH): This indicates the lens power needed to correct nearsightedness (negative value) or farsightedness (positive value). The sphere value is measured in diopters (D).
Cylinder (CYL): This component corrects astigmatism, which occurs when the cornea or lens of the eye is irregularly shaped. The cylinder value is also measured in diopters and may be accompanied by an axis value, indicating the astigmatism's orientation.
Axis: If astigmatism is present, the axis value denotes the angle at which the cylinder power should be oriented to achieve optimal correction.
Addition (ADD): This value is relevant for multifocal or bifocal prescriptions and indicates the additional power needed for reading or close-up tasks.
Pupil Distance (PD): This refers to the distance from one pupil's center to the other pupil's center. This determines where the clearest part of the lens will be.
PRISM: this is used when your eyes need help aligning properly. Prism can shift the images up, down, in, or out to help your eyes focus together, reducing double vision.

Contact Lens Prescriptions

Contact lens prescriptions are tailored specifically for lenses placed directly on the eye's surface. While they contain similar elements to glasses prescriptions, they are often written differently due to the unique characteristics of contact lenses. A contact lens prescription typically includes the following components:

Power (PWR or SPH): Like glasses prescriptions, the power component indicates the strength of the lens needed to correct nearsightedness or farsightedness. However, contact lens powers may differ slightly from glasses powers due to variations in how lenses sit on the eye.
Base Curve (BC): This value represents the curvature of the contact lens and ensures a proper fit on the eye's surface. Base curve values are measured in millimeters and specific to each contact lens type.
Diameter (DIA): The diameter value indicates the size of the contact lens and is also measured in millimeters. It determines how much of the eye's surface the lens covers.

Contact lens prescriptions may also specify the brand and material of the prescribed lenses to ensure compatibility with the wearer's eyes and lifestyle.

How to Read Your Prescription

To better understand the differences between contact lens and glasses prescriptions, let’s breakdown what these numbers and symbols that you see in your prescription mean. Let’s take this prescription for example: -4.25-0.50x180.

The negative symbol here indicates nearsightedness. Meaning you see better up close and need help seeing things at a distance. If you were to see a + symbol, this would indicate farsightedness, meaning that you better far away and objects up close are blurry.
The number following the symbol, in this case 4.25, indicated how strong your prescription is with 0 meaning no correction is needed. The farther your number is from 0, the stronger your prescription is.
The next number, in this case -0.50, indicates your cylinder number, which measures how much astigmatism you have. This means the cornea of your eye has a different curve. In this prescription, the patient has a small amount of astigatism and won’t need correction.
The final number, 180, indicates where the astigmatism is on your cornea. This number is written in degrees from 1-180 and “indicates where the astigmatism lines up.”

Why is there a difference between my glasses and contact lens prescription?

At Downtown Vision Reno, we use what are known as plus cylinder phoropters, the middle number of your prescription will have a plus sign in front of it. These cylinder phoropters, refer to the instrument used to measure the refractive error in your eyes.

Our Carson City office uses minus cylinder phoropters; the middle number in your prescription will have a minus sign. We will have to convert the powers in one notation to another to make sure your prescription is correct.

Because contact lenses sit on your eye, very few contact lens prescriptions will match your glasses prescription due to cylinder correction for astigmatism. If your glasses prescription is -4.25-0.50x180, then the best contact lens for you would be a -4.50DS (DS is diopters of sphere) with no astigmatism correction. In this example, the patient has a small amount of astigmatism that does not need to be corrected.

Most contact lenses are not custom and do not come in every axis (the three numbers at the end). Most come in 10-degree increments, so if your axis is twelve (012), we round it to ten (010). With the standard contact lenses, there are only four or five astigmatism powers (-0.75, -1.25,-1.75, -2.25, and a couple of companies make a -2.75), so we have to round up or down to get your vision to be its best. If your astigmatism exceeds -2.75, we may have to order custom lenses. For example, if your Rx is -4.25-1.00x023, the best lens is -4.50-0.75x020.

As optometrists, we spend years getting this all figured out. It’s not as easy as it seems, and along with assessing your eye health, your best-corrected vision, and all of your options for contact lenses, we charge patients a contact lens fitting fee. Suppose you have high blood pressure and you take medication for it. In that case, your PCP will see you back to ensure the current prescription works correctly before refilling your medication. Thankfully, fitting contact lenses is not a serious situation. Still, poor-fitting lenses can cause damage to your eyes and, in rare cases, have caused permanent scaring and loss of vision.

How long do prescriptions last?

Typically, prescriptions last one to two years. However, many factors can impact this, such as eye health, age, and the type of corrective lens you choose. Even if you believe your vision is perfect and you have no noticeable issues or changes, annual eye exams are crucial for maintaining optimal eye health and detecting potential problems early. Many eye conditions, such as glaucoma, diabetic retinopathy, and macular degeneration, can develop gradually without causing noticeable symptoms in the early stages. Regular eye exams allow your eye care provider to assess your eye health, identify any signs of disease or abnormalities, and intervene promptly if necessary. Additionally, annual exams allow you to update your glasses or contact lenses prescription, ensuring your vision remains clear and comfortable.

The most important thing to remember is that we are here to answer any of your questions and that you don’t have to wait till your next appointment to ask. That’s what we are here for.

Trust us, we’re your eye doctors.

Sources:

https://theeyeavenue.com/understanding-glasses-prescription-vs-contact-lens-prescription/

https://www.hubblecontacts.com/blog/how-to-read-your-contact-lens-prescription

https://www.aao.org/eye-health/glasses-contacts/how-to-read-eyeglasses-prescription