Written by Sonia Kelley, OD, MS on September 15, 2023
A simple way to understand a diopter is to think of it as the lens power of your glasses or contacts. It’s a unit of measurement for how much a curved lens bends light. The number of diopters in your vision prescription indicates the power required to correct your refractive error.
This diopter value is written on an eyeglasses or contact lens prescription. The diopter symbol is D, which comes after the number in your prescription — for example, -2.00 D. A larger number means the lens bends light more, indicating a stronger prescription.
When an eye doctor prescribes you a certain diopter lens, it’s so light coming into your eyes can be focused sharply on your retina, the light-sensitive tissue of the eye. Light focused directly on the retina provides a clear image for the brain to process.
How Does My Eye Doctor Determine What Diopter Lens I Need?
During an eye exam, your eye doctor will ask you, “Which is better, one or two?” This is so they can determine what power lens — measured in diopters — is required to provide you with clear vision.
Eye doctors use a phoropter to test a patient’s refractive error and determine the correct prescription. The phoropter has lenses with various diopter values that may be compared and changed to provide the sharpest vision. This test is called a refraction.
Once the correct power lens for each eye is determined with refraction, the doctor will write the diopter values of the lens on a prescription. This provides the necessary information to the laboratory or dispensary to supply you with the appropriate corrective lenses.
Negative and Positive Diopters
Suppose you can see clearly without any correction. In that case, your eyes probably don’t have a substantial refractive error. In other words, the light entering your eyes lands sharply at a focus on the retina.
But suppose you have a refractive error such as myopia (nearsightedness) or hyperopia (farsightedness). In that case, you will need a curved lens in front of your eyes to focus the incoming light directly on the retina so you can see clearly.
Curved lenses can correct refractive errors such as nearsightedness, farsightedness, or astigmatism. They do this by moving the focus of light closer or farther back so it lands at a sharp point directly on the retina to provide clear vision.
The type and amount of curvature of a corrective lens will depend on the type and severity of the refractive error.
Nearsighted People Need Lenses with Negative Diopter Power
People with myopia need lenses with negative diopter power. In myopia, either your eyeball is too long or your cornea and lens contribute too much focusing power. The result is that incoming light rays come to a focus just in front of the retina and begin to spread again. Since the light rays are dispersed when they land on the retina, a blurry image forms.
Lenses that move the focus of light farther back are called concave lenses or “minus lenses.” Concave lenses are thinner at the center and thicker at the edge. This shape causes them to diverge light. By diverging light, the lenses move the focus farther away, landing it on the retina so that your vision is crisp and clear.
The steeper the curve of the concave lens, the more it diverges the light and the higher the minus power in diopters. A higher negative diopter power is needed for higher myopia because more diverging of light rays is needed.
Farsighted People Need Lenses with Positive Diopter Power
People with hyperopia need lenses with positive diopter power. In hyperopia, your eyeball is too short or your cornea and lens contribute too little focusing power. The result is that incoming light rays will come to a focus behind the retina, forming a blurry image.
Lenses that move the focus of light closer are called convex lenses or “plus lenses.” They are thicker at the center and thinner at the edge. This shape causes them to converge light. By converging light, the lenses move the focus of the light closer up, landing it on the retina so that your vision is crisp and clear.
The steeper the curve of the convex lens, the more it converges the light and the higher the plus power in diopters. A higher positive diopter power is needed for higher hyperopia because more converging of light rays is needed.
In some cases, such as astigmatism, different amounts of power are required in different meridians (axes) in the eye. The diopter value within the lens must vary to correct this type of refractive error. An eye doctor will note this variation in diopter power as the “cylinder” on the prescription. Cylinder refers to the amount of astigmatism correction needed.
Almost everyone develops presbyopia by the time they reach their 50s, making it more difficult to see and read up close. This is because the eye’s natural focusing mechanism, the crystalline lens, becomes harder and loses its capacity to provide focusing power. The eye’s crystalline lens has a convex shape. When you’re younger, it can flex and provide instant extra plus power to help you see clearly when looking at up-close objects.
Once the crystalline lens loses its ability to provide this extra plus power, a convex lens with a plus diopter power must be placed in front of the eye to help you focus up close. This can be in the form of multifocal glasses or contact lenses, or reading glasses.
An eye doctor will note the extra plus diopters as the “add.” The “add” is the positive diopter power added to the distance prescription, which allows you to focus up close. The strength of the add increases as more diopters are required to aid with close-up vision.
An add value will typically range from +0.50 diopters to +3.00 diopters. As you age, you’ll require a higher add value because the crystalline lens progressively loses its flexibility.
People who don’t wear glasses or contact lenses to correct their distance vision may be able to use over-the-counter reading glasses (readers) to see better up close. Follow the directions at the store to determine your add value for readers. Or better yet, discuss with your eye doctor what diopter add value is most appropriate for you.
What Diopter Lens Prescription Do I Need?
An eye doctor can determine what type and power of lens you need to see clearly at a distance and up close. They’ll note it on your eye prescription in units of diopters.
Although the concept of a diopter may seem difficult at first, it’s simply a unit that describes the power of a lens.
A comprehensive eye exam is the best way to determine the diopter power needed in your corrective lenses. During an exam, your eye doctor will also check the health of your eyes so you can maintain clear and healthy vision.
- The diopter. Eye. February 2021.
- What is a diopter? All About Vision. October 2022.
- Diopter and lens power. Michigan Technological University
- Lens power, nominal Power, focal Length, sagittal depth, thick lens power. National Academy of Opticianry. 2020.