Decoding After Dilation: Understanding Your Contact Lens Prescription
No one can see in the dark, unless you have a special super power! But when you're in the dark about your contacts, you don't need a flashlight. You need help reading your contact lens prescription!
All those numbers and letters can tell you a lot about your eyes. You can find out whether you’re near or farsighted. It can also tell you if you have astigmatism and how bad your vision is.
Don't spend any more time in the dark about your eyes. Take a look at our guide to understanding your contact lens prescription below.
What Does My Contact Lens Prescription Mean?
The first part of any prescription for contacts should be your name, the brand and the date it expires. Contact lens prescriptions are good for one to two years. After that, you will not be able to order again until you get another eye exam.
Your prescription for contacts is different than it would be for glasses. So if you’re looking to order glasses too, make sure to get copies of both.
Following your basic identifying information, you'll notice three other parts of the prescription. Each section should have two numbers or letters listed, one for each eye.
The Three Main Parts of a Contact Lens Prescription
To help you see, the contacts must have the right fit, size and strength.
1. The Right Fit (Base Curve)
The right fit for your eyes shows up under the base curve (denoted by BC on the box of contacts). It is how the contact fits your eye. You’ll see this either listed in millimeters or as "steep," "flat" or "median." If the number is low, it means the contact lens needs to have more curvature.
2. The Right Size (Diameter)
The diameter (denoted by DIA on the box of contacts) is how many millimeters across the contact should be. This measurement changes based on how big each person's eye is. This number is usually somewhere around 14mm.
Human eyes are all similar in size no matter how big the person is. They grow to reach full size during puberty. Yet the variation is noticeable enough that everyone requires different-sized contacts.
3. The Right Strength (Power and Diopter)
The power (or sphere) refers to whether you are nearsighted or farsighted. It will have either a minus or a plus in front of the number. If a person is farsighted, or hyperopic, the number will be positive with a plus sign in front. Likewise, nearsighted, or myopic, prescriptions will have a minus sign preceding the number.
The diopter is the strength of your prescription. The letter D on the box of contacts identifies which line lists power and diopter. The number following the minus or plus sign is the diopter, or strength of the prescription.
A person sees objects where light waves converge in front of your eye, called the focal length. The diopter's number tells how far from your eye the light waves will converge, or what your focal length is. The bigger the number, the stronger the prescription is.
The rest of your prescription may be blank. This means you have no further eye conditions that affect how you see. But if you have a condition like astigmatism, you’ll find a little more to your prescription.
With astigmatism the shape of your cornea is a little different. Because the shape isn't a perfect sphere, your eye isn't able to bend light normally to help you see clearly.
A normal contact is shaped to fit an eye with the shape of a soccer ball. Gas permeable and toric lenses are available for eyes with astigmatism, which are shaped more like footballs.
Toric lens prescriptions will show the cylinder and the axis of your eye. On the contact box, both are usually listed on the same line.
The cylinder, always negative, tells how severe your astigmatism is. The axis gives the angle needed to correct your vision. Here, you'll see a number from 0 to 180 degrees.
Ask your doctor about gas permeable and toric lenses. Even more types of contacts for astigmatism are available. Some patients with astigmatism even find colored contacts that fit their eyes. You should be able to find comfortable lenses, despite your condition.
The final listing is for presbyopia. A further diagnosis of presbyopia means a loss of close-up vision, which usually occurs with age. You may need bifocals or other multifocal lenses, which contacts can accommodate.
You'll find the addition listed under the letters ADD on your prescription. The magnification gets a number between 0.5 and 3.0 to tell how severe your presbyopia is.
Doctors select one of your eyes as the dominant eye, which they denote on the prescription. You'll see one of your eyes listed as dominant (D) and one listed as non-dominant (N).
Sometimes people want to change the color of their eyes, but they don't need any vision correction. For color changes only, order non-corrective contact lenses.
Colored lenses with zero corrective power, or plano lenses, are for aesthetic use. They make great additions to costumes for Halloween, theatrical performances or cosplay. They are still a "medical device" though. So a doctor needs to check your eyes before you buy them. Then you'll receive a prescription for these non-corrective lenses.
Plano lens prescriptions will list only the base curve and the diameter because the lenses still need to fit your eyes. However, they won't have any information about the power or diopter because they aren't for correcting vision problems.
Now that you've decoded the numbers, everything is now clear! You'll be able to see through the darkness and read your contact lens prescription.
It doesn't matter what your eye condition is. From astigmatism to presbyopia, there’s likely a contact lens that can work for you. The next time you have an appointment with the optometrist, talk about the numbers you find on your prescription. Then you’ll be all clear about how to place an order for your contact lenses.