An eye prescription can often look like a confusing grid of abbreviations and numbers, but each element provides a specific instruction for manufacturing your corrective lenses. The term “OD” is a Latin abbreviation, Oculus Dexter, designating the measurements for your right eye. The “Axis” is a measurement found in the prescription that relates to a specific type of refractive error, dictating the precise orientation of the corrective power needed in the lens. This number is only included on your prescription if your eye requires correction for non-spherical vision issues.
Deconstructing the Vision Prescription
A standard eyeglass or contact lens prescription organizes information in rows for each eye and columns for different types of correction. The top row uses OD (Oculus Dexter) for the right eye, and the row below it uses OS (Oculus Sinister) for the left eye. The first column, labeled SPH (Sphere), indicates the lens power necessary to correct for simple nearsightedness or farsightedness. A minus sign denotes correction for nearsightedness, while a plus sign signifies farsightedness.
The next two columns, Cylinder (CYL) and Axis, are always paired together because they address a more complex visual issue. The CYL value specifies the magnitude or strength of the lens power needed to correct this specific error. The Axis dictates the exact location where that corrective cylindrical power must be placed on the lens. If the CYL column is blank, the Axis column will also be empty, indicating that a specialized correction is not required.
The Condition Requiring Axis Correction
The presence of a CYL and AXIS value on a prescription signals the existence of a refractive error called astigmatism. This condition is an imperfection in the eye’s curvature, most commonly affecting the cornea. Instead of being perfectly spherical, an eye with astigmatism is shaped more like the side of a football. This oblong shape means the eye has two principal curves, one steeper than the other, which are 90 degrees apart.
This irregular curvature prevents light from focusing evenly onto the retina. Because the light is bent differently along different meridians of the eye, it results in blurred, stretched, or distorted vision at any distance. Standard spherical lenses cannot fully correct this uneven focusing, which is why a separate measurement is required.
Defining the Orientation of the Axis
The Axis number specifies the precise orientation of the steepest curve on the astigmatic eye, which is the direction the corrective lens must target. This value is measured in degrees and ranges from 1 to 180. This scale acts like a protractor superimposed over the eye, providing the optical laboratory with the exact angle for lens fabrication.
A measurement of 90 degrees corresponds to the vertical meridian of the eye, while 180 degrees corresponds to the horizontal meridian. If your Axis is listed as 45, the steepest curve is oriented diagonally between the horizontal and vertical planes. The corresponding cylindrical lens power is then positioned along this exact angle to neutralize the uneven curvature. Accurate measurement of the Axis is necessary because even a slight rotational error in the finished lens can lead to eye strain and blurred vision.