How to Read Your Optical Prescription: SPH, CYL, Axis, ADD & Pupillary Distance (PD)

Optometry retains classical Latin notation to eliminate confusion across international medical records:

• OD (Oculus Dexter): Your Right Eye.
• OS (Oculus Sinister): Your Left Eye.
• OU (Oculus Uterque): Both Eyes (used when a recommendation applies equally to both eyes).

On modern prescription slips, you may see 'RE' (Right Eye) and 'LE' (Left Eye), but OD and OS remain the standard legal designations used by optical laboratories.

Sphere indicates the amount of optical lens power required to correct your basic near- or farsightedness across all meridians of the eye. It is measured in Diopters (D) in steps of 0.25 (e.g., -1.25, -2.50, +3.75):

• Minus Sign (-): Indicates Nearsightedness (Myopia). You see clearly close up, but distant road signs, movies, and faces are blurred.
• Plus Sign (+): Indicates Farsightedness (Hyperopia). Distant vision is easier, but reading and close tasks require exhausting focusing effort.
• PL or Plano: Zero corrective power. The eye requires no spherical diopters for distance vision.

If you have an asymmetrical corneal curvature (astigmatism), your prescription will list two corresponding values:

• Cylinder (CYL): The amount of lens power needed to correct the astigmatic differential between your steepest and flattest corneal meridians. In modern optometry, CYL is almost universally written with a minus (-) sign. If this box is blank, 0.00, or marked SPH, you have no significant astigmatism.
• Axis: A number between 1 and 180 degrees. It does not indicate lens strength; it defines the exact rotational angle at which the cylindrical correction must be surfaced into the lens. An axis of 180° is purely horizontal; 90° is purely vertical.

Example: -2.00 SPH / -0.75 CYL x 180 AX means you have 2 diopters of myopia, with an additional 0.75 diopters of astigmatism aligned horizontally along the 180-degree meridian.

ADD stands for 'Additional Power.' It represents the extra magnifying plus-power required on top of your distance prescription to allow comfortable close-up reading past the age of 40 (the onset of presbyopia).

ADD values typically range from +0.75D to +3.00D, and the value is almost always identical for both eyes. This number is used when manufacturing bifocals, reading glasses, and digital progressive lenses.

The single most critical number often left off basic prescription cards is your Pupillary Distance (PD)—the distance in millimeters between the centers of your two pupils. In adults, PD typically ranges between 58mm and 70mm.

Every prescription lens possesses an optical center (OC)—the precise geometric point on the glass where incoming light passes through without prismatic deviation. If your glasses are manufactured with a PD of 62mm, but your actual eyes have a PD of 66mm, your pupils are forced to look through the off-center peripheral zone of the lens.

In accordance with Prentice's Rule (P = c × D), looking off-center through a powered lens induces an artificial optical prism. This pulls your eyes out of alignment, causing persistent eye strain, double vision, nausea, and severe headaches. In precision optical manufacturing, monocular PD (measured separately from the nasal bridge to each pupil) must be verified within ±0.5 mm tolerance.