The contact lens prescription you receive from an eye care professional contains several abbreviations and values that determine how your vision is corrected. The Sphere (often abbreviated as SPH or PWR) is the most fundamental measurement, representing the primary strength of the lens. This value is the core component that corrects the most common forms of focusing errors in the eye, providing clear vision.
Defining the Sphere Value in Contact Lenses
The Sphere value represents the optical power necessary to correct a refractive error in the eye. This power is measured in diopters (D), which quantifies the lens’s ability to bend light rays. Refractive errors occur when the eye’s shape prevents light from focusing precisely onto the retina, which is necessary to create a sharp image.
The SPH value dictates the strength of the lens required to adjust the incoming light path, ensuring it converges exactly where it should. The numerical part of the sphere value indicates the magnitude of the correction needed.
A higher number, regardless of whether it is positive or negative, signifies a stronger vision correction is required. For example, a prescription of -4.00 D is twice as strong as a -2.00 D prescription. Contact lens manufacturers typically produce lenses in increments of 0.25 D, allowing for precise customization.
Interpreting Sphere: The Role of Plus and Minus Signs
The sign preceding the number in the sphere value indicates the type of refractive error being corrected. This sign reflects where the light is currently focusing within the eye.
A negative sphere value, indicated by a minus sign (-), corrects for myopia (nearsightedness). In a myopic eye, light focuses slightly in front of the retina, blurring distant objects. The contact lens used for this correction is concave, meaning it is thinner in the center, and it works by causing the light rays to diverge slightly.
Conversely, a positive sphere value, marked by a plus sign (+), corrects for hyperopia (farsightedness). This condition occurs when the eye is too short, causing light to focus theoretically behind the retina. The lens used is convex, thicker in the center, and serves to converge the light rays forward onto the retinal surface.
Sphere vs. Astigmatism Correction
The Sphere value provides a uniform corrective power across the entire surface of the lens, assuming the eye is perfectly spherical. This works well for eyes where the refractive error is consistent in all directions, such as simple nearsightedness or farsightedness. The spherical lens design bends light equally along every meridian.
However, many eyes are not perfectly spherical; instead, they are shaped more like a football, a condition known as astigmatism. This irregular curvature requires a different level of correction in different directions. For these eyes, a second set of values is included on the prescription: Cylinder (CYL) and Axis.
The Cylinder value specifies the extra power needed to compensate for the astigmatism, which is added only along a specific meridian. The Axis value, measured in degrees from 1 to 180, defines the precise orientation of that astigmatic curvature on the eye. The Sphere value provides the base, uniform power, while the Cylinder and Axis work together to introduce a targeted, non-uniform correction.
Practical Impact of Incorrect Sphere Power
Wearing contact lenses with an incorrect Sphere value can immediately affect visual comfort and clarity. Persistent blurred vision, especially at a distance, is the most common sign that the lens power is wrong. The eyes constantly struggle to compensate for the focusing error, which leads to significant eye strain.
This continuous effort to focus can trigger secondary symptoms like frequent headaches and general visual fatigue. A substantially incorrect power may also cause temporary disorientation or dizziness. Note that the power of over-the-counter reading glasses is not interchangeable with the SPH value on a contact lens prescription, as the two lenses sit at different distances from the eye.
Routine eye examinations are necessary to ensure the sphere value is precisely matched to the eye’s current needs. While an incorrect power does not cause permanent damage, it compromises visual acuity and comfort. Using the Sphere value prescribed by an eye care professional ensures the lens provides optimal light focus.