The term “vision number” encompasses two concepts: a fractional measurement reflecting the sharpness of your sight (visual acuity), and numerical values on a prescription slip quantifying the optical power needed in a corrective lens. Understanding both measurements is necessary to grasp the state of your eyesight and the mechanics of your corrective eyewear. These numbers are determined through a comprehensive eye examination, which assesses the eye’s ability to focus light onto the retina. They form a standardized language used by eye care professionals globally to define your specific visual needs.
Decoding Visual Acuity: The 20/20 Standard
Visual acuity is a measurement of how clearly you can discern shapes and details at a given distance. This measurement is typically represented as a fraction, with the most commonly recognized standard being 20/20. The system is based on the Snellen chart, a series of standardized letters that decrease in size down the chart.
The top number, 20, represents the testing distance in feet. The bottom number represents the distance at which a person with statistically normal vision could clearly read the same line of letters. For example, 20/20 vision means you see at 20 feet what a normal eye sees at 20 feet.
If your visual acuity is recorded as 20/40, it indicates that you must be 20 feet away to clearly see a letter that a person with normal vision could distinguish from 40 feet away. The larger the bottom number, the less sharp your distant vision is without correction. Vision worse than 20/200 is often associated with the definition of legal blindness in the United States.
In countries using the metric system, this measurement is frequently expressed as 6/6, where the 6 represents the testing distance in meters. Both 20/20 and 6/6 describe the same level of visual performance, representing the standard reference point for normal, uncorrected vision. This fractional notation provides a simple comparison of a person’s resolving power against an established norm.
Understanding Prescription Components and Diopters
The specific strength of a corrective lens is measured in units called diopters (D), which quantify the optical power of the lens. A diopter value indicates the degree to which a lens bends light to achieve focus on the retina. A higher absolute diopter number signifies a stronger lens power is required to correct the refractive error.
The Sphere (SPH) value corrects for the general refractive error of nearsightedness or farsightedness. This spherical correction is necessary when the eye is either too long (nearsightedness) or too short (farsightedness), causing light to focus in front of or behind the retina, respectively.
The Cylinder (CYL) value addresses astigmatism, a condition where the cornea or lens is shaped more like a football than a sphere. This irregular curvature causes light to focus unevenly, resulting in blurred vision at any distance. The CYL number specifies the amount of extra lens power needed to compensate for this unevenness. If this column is blank or marked with “DS” (diopter sphere), no significant astigmatism correction is needed.
The Axis (AXIS) value is a number from 1 to 180 degrees that accompanies the CYL correction, defining the orientation of the astigmatism on the eye. The Axis number tells the lens fabricator precisely where to place the cylindrical power for optimal visual correction. The cylinder and axis values always appear together, as one is meaningless without the other.
Locating and Interpreting Your Vision Numbers
When examining your prescription slip, identify the abbreviations specifying which eye is referenced. The Latin abbreviations OD, or oculus dexter, refers to the right eye, and OS, or oculus sinister, refers to the left eye. Occasionally, OU, or oculus uterque, may indicate both eyes share the same measurement.
The sign preceding the SPH number reveals the type of general vision problem being corrected. A negative sign (–) indicates myopia, or nearsightedness, meaning distant objects appear blurry. This signifies the lens is concave, designed to diverge light rays to push the focal point back onto the retina.
Conversely, a positive sign (+) indicates hyperopia, or farsightedness, where close objects may appear blurred. This signifies the lens is convex, designed to converge light rays to pull the focal point forward onto the retina. If there is no sign, the number is assumed to be positive.
For individuals over the age of 40 who require help focusing on near objects due to presbyopia, a separate value labeled ADD, or Addition, may be present. This number, always positive, represents the extra magnifying power needed for reading or other close-up tasks. The ADD value is combined with the distance prescription to create the lower portion of a bifocal or progressive lens.