Your eye doctor's prescription may look like a foreign language at first glance. With terms like SPH, CYL, and AXIS, it's natural to feel confused. However, understanding your eyeglass prescription is essential for getting glasses that will truly improve your vision and daily life. This guide breaks down each number and term, transforming that puzzling document into a clear roadmap for your next eyewear purchase.

The Right and Left of It: OD & OS

Every prescription starts by distinguishing between your eyes. OD stands for oculus dexter, Latin for right eye. OS stands for oculus sinister, meaning left eye. You might also see OU (oculus uterque), which refers to both eyes. Your prescription lists specific values for each eye since they often differ.

SPH (Sphere): The Core of Your Vision

The SPH (Sphere) value is the main measurement of your vision. It shows the lens power needed to correct nearsightedness or farsightedness.

• A minus sign (-) indicates nearsightedness, meaning you struggle to see distant objects. The higher the number after the minus sign, the stronger your prescription.
• A plus sign (+) or no sign indicates farsightedness, meaning you have difficulty seeing things up close.

This number forms the foundation of your prescription as it defines the primary correction your vision needs.

CYL (Cylinder) & AXIS: For Astigmatism

If your prescription shows numbers in the CYL (Cylinder) and AXIS columns, you have astigmatism—a common condition where an irregularly shaped cornea causes blurry or distorted vision at all distances.

• The CYL value measures the lens power needed to correct astigmatism. A higher number indicates more severe astigmatism.
• The AXIS value, a number between 1 and 180, indicates the orientation of the astigmatism. This is crucial for correctly positioning the lens correction.

If these columns are empty, you don't have astigmatism and can disregard them.

ADD (Addition): The Secret to Close-Up Vision

The ADD (Addition) value is primarily for people who need bifocal or progressive lenses. This number represents the extra magnifying power needed for reading and close-up tasks. It addresses presbyopia, a natural age-related condition that makes focusing on near objects difficult. The ADD value is always positive and typically the same for both eyes.

PD (Pupillary Distance): A Critical Measurement

The PD (Pupillary Distance) measures the space between your pupil centers in millimeters. This measurement is vital for aligning the optical center of your lenses with your pupils. Without an accurate PD, your glasses may not provide clear vision, regardless of how precise the other prescription values are. While most prescriptions include this measurement, if yours doesn't, we offer a simple tool on our site to measure it yourself.

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