When looking at a contact lens prescription, SPH stands for “Sphere.” This term represents the primary indicator of the lens’s strength and the overall correction needed for your vision. The SPH value dictates the power required to refocus light precisely onto the retina. This numerical value addresses the most common refractive errors.
Defining Spherical Power
SPH defines the spherical component of the lens power, which is the uniform corrective strength applied across the entire optical zone of the contact lens. This power corrects the eye’s primary refractive error, ensuring light entering the eye is bent correctly. The power is measured in a unit called a Diopter (D), which quantifies the optical power of a lens. Lenses are typically manufactured in increments of 0.25 Diopters. A spherical lens has the same curvature in all meridians, meaning the light-bending capability is consistent across the entire surface. This uniform curve is sufficient for people whose eyes lack significant astigmatism.
Interpreting the Values
The SPH value is presented as a number always preceded by either a minus sign (-) or a plus sign (+). The sign indicates the direction of correction needed, while the magnitude of the number signifies the severity of the refractive error. A value closer to zero, such as -0.75 D, represents a mild need for correction. Conversely, a value farther from zero, like +4.50 D, indicates a more significant refractive error requiring a stronger lens. The numerical value measures the lens’s ability to converge or diverge light rays to compensate for the eye’s focusing error.
Correcting Common Vision Conditions
The sign preceding the SPH number correlates with the type of vision condition being corrected. A negative SPH value is prescribed for Myopia (nearsightedness), where distant objects appear blurry because light focuses in front of the retina. The minus sign denotes a diverging lens needed to push the focal point backward onto the retina.
Conversely, a positive SPH value corrects Hyperopia (farsightedness), where the eye focuses light behind the retina. The plus sign indicates a converging lens that pulls the focal point forward to land correctly on the retina. For individuals without significant corneal irregularity, the SPH value is the sole determinant of the lens power.
Why Contact SPH Differs from Eyeglass SPH
The SPH value on a contact lens prescription often differs from that on an eyeglass prescription, especially for stronger powers. This difference is due to “vertex distance.” Eyeglasses sit 10 to 14 millimeters away from the eye, while a contact lens rests directly on the cornea.
This change in physical distance alters the effective power of the correction. For higher prescriptions (exceeding 4.00 Diopters), a calculation must be performed to adjust the SPH power. Because of this necessary conversion, you should never purchase contact lenses using the SPH value found on your eyeglass prescription.