When you receive an eyeglass prescription, the sheet of paper often appears to be a cryptic array of letters and numbers. This standardized document is a detailed formula created by your eye care professional to correct any refractive error that prevents light from focusing perfectly onto your retina. Understanding this formula involves breaking down the medical abbreviations and numerical values that dictate the specific power and orientation of the lenses you require. The prescription translates complex visual requirements into an actionable plan for lens fabrication.
Decoding the Eye Labels: OD, OS, and OU
The first step in deciphering any eye prescription is understanding the foundational Latin abbreviations that specify which eye the measurement applies to. The abbreviation OD stands for Oculus Dexter, which designates the measurements for your right eye. The measurements for your left eye are represented by OS, the Latin abbreviation for Oculus Sinister. Since eyes often have different vision needs, these two columns are listed separately. A third, less frequently used term is OU, which stands for Oculus Uterque, signifying both eyes. This abbreviation is typically reserved for instances where the prescription is exactly the same for both eyes.
The Core Vision Measurements: SPH, CYL, and AXIS
The largest part of your prescription details the primary lens powers required to bring distant objects into focus. This correction begins with the SPH, or Sphere, column, which measures the main power of the lens in units called diopters (D). The spherical value addresses the overall focusing power needed to correct either nearsightedness or farsightedness.
The sign preceding the number in the SPH column is crucial for determining the type of refractive error being corrected. A negative sign (-) indicates myopia (nearsightedness), where light focuses in front of the retina. Conversely, a positive sign (+) denotes hyperopia (farsightedness), which occurs when light focuses behind the retina. The power listed is the uniform correction needed across the entire lens surface.
The second primary column, labeled CYL or Cylinder, indicates the lens power required to correct astigmatism. Astigmatism is a common condition where the cornea or lens is imperfectly curved, resembling a football shape rather than a perfect sphere. If this column is left blank or contains “0.00,” it means any existing astigmatism is minor enough not to require correction.
The CYL value specifies the amount of extra power needed to compensate for this irregularity, and it must be paired with the AXIS value to be effective. The Axis measurement is a number between 1 and 180 degrees that indicates the precise orientation of the astigmatism on the eye. This number tells the lens maker where to place the cylindrical correction on the lens to align with the eye’s irregular curve.
Understanding Specialized Corrections: ADD Power and Prism
Some prescriptions include specialized values for near vision and eye alignment. The ADD, or Addition, column is a common feature for individuals who require multifocal or progressive lenses. This value represents the extra magnifying power added to the lower portion of the lens to help with reading and close-up tasks.
The need for ADD power is generally related to presbyopia, an age-related condition where the eye’s natural lens loses flexibility, making it difficult to focus on near objects. This value is always positive and is typically the same for both the right and left eyes, often ranging between +0.75 D and +3.50 D.
A less common correction is the Prism value, which is used to treat eye muscle imbalances, also known as strabismus. The prism works by shifting the image seen by one eye so that it aligns correctly with the other, allowing both eyes to work together comfortably. This measurement is noted in prism diopters and is followed by a base direction, such as BU (Base Up), BD (Base Down), BI (Base In), or BO (Base Out), which indicates the orientation of the prism’s thickest edge.
Interpreting Severity and Common Conditions
The numerical values in the SPH column are a direct indicator of the degree of myopia or hyperopia. For nearsightedness, a prescription between -0.50 D and -3.00 D is considered mild, while values ranging from -3.25 D to -5.00 D are classified as moderate. A value exceeding -5.25 D represents a higher degree of myopia, requiring significantly stronger corrective lenses.
For farsightedness, a spherical power from +0.25 D to +1.00 D is considered mild, with moderate levels beginning around +3.00 D and above. The higher the absolute number, whether positive or negative, the greater the degree of refractive error and the more dependent the individual is on corrective lenses for clear vision.
The magnitude of the CYL number correlates directly with the severity of astigmatism. A cylinder correction up to 2.50 D is considered a moderate level of astigmatism. Values higher than 2.50 D suggest a more pronounced irregularity in the corneal shape, demanding stronger cylindrical lens power.