Understanding the Contact Lens Prescription

Young female doctor with laptop writing notes on desk in clinic

While your contact lens and glasses prescription may look similar, they are not interchangeable. Your contact lens prescription is based on the characteristics of your eyes, while your glasses prescription is based on the distance between your eyes and the lenses. A contact lens prescription includes the power of your lenses and the curvature of your cornea. A glasses prescription includes only the power of your lenses.

Contact lenses prescriptions include a variety of different specifications that are not part of the standard glasses prescription, and these can include:

The Base Curve

The base curve is the curve of the back surface of a contact lens. It is measured in millimeters and has a range of values from 8.4 mm to 10.0 mm. The base curve determines the amount of curvature on the front surface of the lens, which in turn affects how the lens fits onto your eye. A steeper base curve value produces a more curved front surface, and a flatter base curve value produces a less curved front surface. The flatter the base curve, the more “ball-like” your eye appears. The standard or default base curve for most lenses is 8.6 mm. However, depending on the actual shape of your eye, you may require a different base curve to get the right lens fitting. Your doctor can determine your specific base curve needs during a comprehensive eye exam.

The Lens Diameter

The lens diameter, also known as the optic zone diameter, is the measurement of the total curvature of your contact lens. It determines how wide the central portion of your lens will be and directly impacts how well the lens will fit in your eye. A smaller lens diameter means less overall bearing surface on the eye and can cause uncomfortable fitting issues such as poor centration or frequent dislodging. On the other hand, a larger diameter can cause bigger lenses that may be visible when you blink or at rest. Generally, most people have a safe and comfortable middle ground between 13.5 to 14.5 mm. However, this may vary depending on your eye’s specific shape and size.

In addition to base curve and diameter, a contact lens prescription may include an axis and cylinder for toric lenses or an add for multifocal lenses. The axis indicates the degree of astigmatism, while the cylinder is used to correct it. This add is used to correct presbyopia, the age-related loss of near vision. These are the only two measurements found on both contact lenses and glasses prescriptions.

Toric contact lenses are designed to correct for astigmatism and presbyopia, the age-related loss of near vision. The cylinder number mentioned on a contact lens prescription always denotes the correction for astigmatism and is given as a minus number. On a glasses prescription, the cylinder value can be plus or minus but will be blank if there is no astigmatism. The same goes for the axis values. If in case you have a high degree of astigmatism, you may need to wear gas-permeable contact lenses or custom contact lenses that are made to fit your specific corneal shape.

The Material of the Lens

The material of the lenses is important for a few reasons. The weight of the lens is a big factor, as heavier lenses can cause headaches and neck pain. Glass lenses are much heavier than plastic lenses, which is why most glasses nowadays are made of plastic. Contact lenses are usually made of hydrogels, a type of polymer that can absorb water. Hydrogels are used because they are very soft and comfortable to wear. They also allow oxygen to pass through, which is important for the health of your eyes.

Types of Contact Lenses

Soft Contact Lenses

They are made from a soft, pliable material that conforms to your eye’s shape and is available in a wide range of prescription strengths. Soft lenses are also available in a variety of colors and styles, including options that can enhance or change your natural eye color.

Rigid Gas Permeable (RGP)

Rigid gas permeable (RGP)  contact lenses are made of a brittle, oxygen permeable material that resists deposits. They produce clear, sharp vision and provide excellent comfort for patients who adapt to them. An RGP lens is usually larger in diameter than a soft lens, so it covers more of the cornea. This can provide better protection from foreign bodies and irritants such as pollen and dust. RGPs must be custom fit to each patient’s eye and require some adaptation time before they are comfortable. RGPs are also generally more durable than soft lenses, so they can be a good choice for active patients or those with allergies or other conditions that cause them to rub their eyes frequently.

Hybrid Contact Lenses

Hybrid contact lenses are a type of lenses that offer a combination of the best features of both hard and soft lenses. Hybrid contact lenses offer the benefits of both types of lenses, making them a popular choice for many people. They’re made of RGP material in the center, which provides clear vision, and soft material around the edges, which is comfortable to wear. They’re easier to adjust to than hard lenses and may be a good option for people who have had difficulty wearing other types of lenses.  Here https://www.contactlenses.co.uk, you can find all types of contact lenses.

Can You Convert a Glasses Prescription To Contact Lenses?

If you wear glasses and are considering contact lenses, you may wonder if you can use the same prescription. The answer depends on a few factors. It is essential to note that the eyeglass prescription reflects the amount of correction needed for distance vision.

In contrast, the contact lens prescription reflects the amount of correction needed for up-close vision. As a result, it is, therefore, necessary to have a stronger prescription for contact lenses than for glasses. Additionally, contact lenses sit directly on the eye, requiring a different curvature than glasses to fit properly.

For these reasons, it is usually impossible to simply convert a glasses prescription to a contact lens prescription. As a result, the two types of prescriptions are not directly comparable. If you need a contact lens prescription, it is always best to visit an eye doctor so that they can properly measure your eyes and prescribe the correct lenses.