What is a Diopter

What is a Diopter

Confused about what diopters are and how they affect your vision? Our latest blog post explains everything you need to know in simple terms.

Ever wondered how your glasses or contact lenses correct your vision? It all comes down to diopters. These numerical measurements determine the power of the lenses and how they bend light.

When you receive your vision prescription, you'll notice a diopter value. Represented by the symbol D, it indicates the strength of the lenses needed to correct any refractive errors. A higher number means a more powerful prescription, signifying a greater ability to bend light.

By prescribing the appropriate diopter lens, your eye doctor helps focus light precisely onto your retina. Think of the retina as the light-sensitive tissue in your eye that captures images. By ensuring the light is correctly focused on the retina, you can enjoy a clear and sharp visual experience.

When you visit your eye doctor for an eye exam, they will assist you in finding the perfect lens for your vision. Using a device called a phoropter, they will ask you a series of questions, such as "Which is better, one or two?" This allows them to accurately determine the strength of the lens you need, measured in diopters, to ensure clear vision.

How Does My Optometrist Establish the Diopter Lens Requirement?

When you visit your eye doctor for an eye exam, they will assist you in finding the perfect lens for your vision. They will ask you a series of questions, such as "Which is better, one or two?" This allows them to accurately determine the strength of the lens you need, measured in diopters, to ensure clear vision.

During the exam, the eye doctor will conduct a test called refraction. This involves using the phoropter, which contains lenses with various diopter values. By comparing and adjusting these lenses, they can determine the exact prescription that will provide you with the clearest vision.

Once your doctor has determined the correct lens power for each eye through the refraction test, they will record the diopter values on a prescription. This information is then sent to the lab or dispensary, where they will create the appropriate corrective lenses to meet your needs.

Understanding Negative and Positive Diopters

Having clear vision without any correction is a sign that your eyes do not have a significant refractive error. In simpler terms, this means that the light entering your eyes focuses perfectly on the retina, resulting in sharp vision.

However, if you have a refractive error like myopia (nearsightedness) or hyperopia (farsightedness), you will need a curved lens in front of your eyes to focus the incoming light directly onto the retina, ensuring clear vision.

These curved lenses, also known as diopters, can correct refractive errors such as nearsightedness, farsightedness, or astigmatism. They achieve this by adjusting the focal point of light to land precisely on the retina, thus ensuring clear vision.

The specific type and degree of lens curvature necessary depend on the type and severity of your refractive error.

Nearsightedness Requires Negative Diopter Lenses

If you're nearsighted, your eyeball may be elongated or your cornea and lens may be too powerful, causing light rays to converge in front of the retina. This leads to blurry vision. That's where concave or "minus lenses" come in. These lenses shift the focal point farther back, guiding the light to diverge and land on the retina. The result? Clear and sharp vision.

To achieve the desired light divergence, the curve steepness of the concave lens determines the negative diopter power needed. The more myopia you have, the greater the divergence of light rays, requiring a higher negative diopter power.

Farsightedness Requires Positive Diopter Lenses

On the other hand, if you're farsighted, your eyeball may be shorter, or your cornea and lens may lack focusing power, causing light rays to converge behind the retina. This also creates a blurry image. But don't worry because convex or "plus lenses" are here to help. These lenses converge the light, bringing the focal point closer and ensuring clear and precise vision.

Just like with concave lenses, the curve steepness of the convex lens affects the degree of light convergence and the positive diopter power needed. The more severe your hyperopia is, the greater the convergence of light rays, requiring a higher positive diopter power.


When it comes to astigmatism, the power needed by the eye varies in different axes. To address this refractive error, the lens diopter values must be adjusted accordingly. Experts refer to this as the "cylinder" on your prescription, indicating the exact amount of correction required for astigmatism. 


As we age, our ability to see and read up close can become more challenging. This is due to a natural change in the eye's focusing mechanism known as presbyopia. The crystalline lens, responsible for clear vision up close, becomes less flexible with time, causing a decline in its focusing power.

To compensate for this loss, a convex lens is often needed to assist with close-up focus. This can be achieved through the use of multifocal glasses or contact lenses or by simply using reading glasses.

During your consultation with an eye doctor, they will discuss the additional plus diopters, known as the "add," required to enhance your close-up vision. The strength of the add will depend on the severity of your presbyopia, with values ranging from +0.50 diopters to +3.00 diopters.

As you age, the add value may increase as the flexibility of the crystalline lens continues to decline. If you don't currently wear glasses or contact lenses, over-the-counter reading glasses can also be used to improve your close-up vision. Consult the store's guidelines or speak with your eye doctor to determine the appropriate add value for your specific needs.

What Diopter Prescription Should My Lenses Have?

With the help of your eye doctor, you can easily determine the exact lens type and power needed for clear vision, whether for up close or distance viewing. This information will be written down in your eye prescription using diopters.

A comprehensive eye exam is crucial to accurately determine the precise diopter power your corrective lenses should have. This exam also allows your eye doctor to thoroughly assess the health of your eyes, ensuring that your vision remains crystal-clear and healthy. So, why wait? Schedule an eye exam today for peace of mind.

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