Reading a Prescription
A prescription tells you the key data for making glasses: your refractive status, the direction of your astigmatism, your pupillary distance, and more.
- SPH (sphere): a positive value indicates hyperopia or presbyopia, a negative value indicates myopia.
- CYL (cylinder): the astigmatism power, usually negative; the larger the magnitude, the stronger the astigmatism.
- AX (axis): the axis of the astigmatism (0°-180°), indicating the direction that has no refractive power.
- PD (pupillary distance): the interpupillary or monocular distance, which affects where the optical center of the lens sits.
- ADD (add power): the extra near power for presbyopia, used in progressive and bifocal designs.
Try to interpret the following prescriptions, then compare yourself with the reference answers.
Exercise 1: OD -1.50 / DS, PD 64
Reference Answer: The right eye is myopic by 150 degrees, with no astigmatism, and the pupillary distance is 64mm.
Tip: / DS means the cylinder is zero, that is, there is no astigmatism.
Exercise 2: OS +2.00 / +0.75 × 90, ADD +1.50
Reference Answer: The left eye is hyperopic by 200 degrees, with 75 degrees of hyperopic astigmatism at axis 90°; the add power of +1.50 is used for near-vision reading (progressive/bifocal lenses).
Tip: A positive SPH indicates hyperopia; ADD appears only when presbyopia needs a near addition.
Exercise 3: OD -3.00 / -1.00 × 180, OS -2.75 / -0.75 × 175, PD 62
Reference Answer:
- The right eye is myopic by 300 degrees, with 100 degrees of astigmatism at axis 180°.
- The left eye is myopic by 275 degrees, with 75 degrees of astigmatism at axis 175°.
- The pupillary distance is 62mm.
Interpretation: The prescriptions of the two eyes are close (a difference of ≤ 0.25D), which is normal symmetric refraction; the astigmatism axes are close to horizontal (180°/175°), so this is regular astigmatism, which glasses can correct precisely.
Reading a prescription accurately helps you discuss your lens needs with the optometrist and reduces discomfort.