Understanding the Basics of an Eye Prescription
An eye prescription is a written record detailing the specific corrective lenses needed for optimal vision. It guides opticians in crafting glasses or contact lenses.
The standard unit of measurement for lens power in an eye prescription is the diopter. This unit quantifies the optical power of a lens, indicating its ability to converge or diverge light. A higher diopter number, whether positive or negative, signifies a greater degree of vision correction is necessary.
Decoding Your Prescription Numbers
Your eye prescription contains several abbreviations, each representing a distinct aspect of your vision correction. Understanding these terms helps clarify the specific nature of your visual needs.
The “SPH” or Sphere value indicates the main lens power required to correct nearsightedness (myopia) or farsightedness (hyperopia). A negative number in this column points to nearsightedness, meaning distant objects appear blurry, while a positive number indicates farsightedness, where close-up objects may be unclear. This value is uniform across the entire lens.
“CYL” stands for Cylinder, and this number addresses astigmatism, a common condition causing blurred vision at any distance due to an irregularly shaped cornea or lens. If this column is blank, it means you do not have astigmatism or it is not significant enough to require correction.
The “AXIS” number accompanies the cylinder power, measured in degrees from 1 to 180. It specifies the precise orientation of the cylinder correction on the lens.
“ADD,” or Addition, power is found in prescriptions for bifocal, trifocal, or progressive lenses. This positive number represents the magnifying power added to the distance prescription for reading or other near-vision tasks. It primarily addresses presbyopia, the age-related loss of near focusing ability.
Lastly, “PRISM” indicates the amount of prismatic power necessary to correct specific eye alignment issues. This correction is less common and is prescribed when the eyes do not work together perfectly, which can cause double vision.
What Different Strengths Indicate
The numerical values on your eye prescription directly correlate with the degree of vision correction required. For instance, a larger negative Sphere (SPH) number indicates a more pronounced degree of nearsightedness. Someone with a -4.00 SPH prescription experiences more significant difficulty seeing distant objects clearly compared to someone with a -1.00 SPH.
Conversely, a higher positive SPH value denotes a greater extent of farsightedness. An individual with a +3.00 SPH prescription struggles more with near vision than someone with a +0.75 SPH.
The magnitude of the Cylinder (CYL) number reflects the severity of astigmatism. A larger CYL value, such as -2.00, signifies a more significant irregular curvature of the eye, demanding a stronger cylindrical correction than a smaller value like -0.50.
The Addition (ADD) power also reflects a specific need, mainly for near vision correction. As people age, the eye’s natural lens stiffens, making it harder to focus on close objects. A higher ADD power, for example, +2.50, suggests a greater need for magnification to comfortably read or perform other close-up tasks compared to a lower ADD power, like +1.00.
Beyond the Numbers: The Eye Exam Process
The precise numbers on an eye prescription are the result of a comprehensive eye examination conducted by a qualified eye care professional. During this process, an optometrist or ophthalmologist employs various tests to accurately measure your refractive errors.
One primary test involves the use of a phoropter, an instrument with multiple lenses through which you look. The eye care professional systematically changes these lenses while asking for feedback on clarity, precisely determining the optimal lens power. This refraction test is central to identifying the Sphere and Cylinder values.
Visual acuity charts, such as the Snellen chart, are also utilized to assess how clearly you see at various distances. By combining the objective measurements from instruments with your subjective responses, the eye care professional tailors a prescription that is unique to your eyes. Regular eye exams are important for monitoring changes in vision and ensuring your prescription remains accurate.