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Reading an eyeglass prescription is one of the most fundamental skills you will use as a paraoptometric. You will encounter prescriptions dozens of times per day -- when entering orders into the lab system, when verifying finished glasses against the Rx, when patients call asking about their prescription, and when the doctor hands you a script to process. Being able to read, interpret, and explain every component of a prescription is not optional. It is expected.
This guide breaks down every element of a standard eyeglass prescription, explains what each value means clinically, and walks through real-world examples. If you are studying for the CPO or CPOA exam, prescription reading appears in the Ophthalmic Optics and Dispensing domain and connects to Refractive Status questions. Expect multiple exam questions that present a prescription and ask you to interpret it.
Even experienced paraoptometrics sometimes second-guess unusual prescriptions. This guide gives you the foundation to read any Rx with confidence, whether it is a simple single vision script or a complex multifocal with prism.
A standard eyeglass prescription has a consistent structure regardless of which doctor writes it. The values are organized in a table format with the right eye (OD) on the first line and the left eye (OS) on the second. Each line contains some or all of the following components:
| Component | What It Means | Typical Range |
|---|---|---|
| Sphere (Sph) | Main focusing power. Corrects myopia (-) or hyperopia (+) | -20.00 to +20.00 D |
| Cylinder (Cyl) | Astigmatism correction. Additional power in one meridian only | -0.25 to -6.00 D (typical) |
| Axis | Orientation of cylinder correction in degrees | 1 to 180 degrees |
| Add | Extra plus power for reading (presbyopia correction) | +0.75 to +3.50 D |
| Prism | Light displacement for eye alignment issues | 0.5 to 10 prism diopters |
| PD | Pupillary distance for optical center placement | 54 to 74 mm (adults) |
The sphere value is the main corrective power of the lens. It tells you how much the lens needs to converge or diverge light to bring the focal point onto the retina. Sphere is measured in diopters (D) and is always written first on the prescription line.
A negative sphere value (e.g., -3.50) means the patient is myopic. Their eye focuses light in front of the retina, so they need a diverging (minus) lens to push the focal point back. The higher the minus number, the more myopic the patient. Someone with -1.00 has mild myopia; someone with -8.00 has high myopia and likely cannot function without correction.
A positive sphere value (e.g., +2.25) means the patient is hyperopic. Their eye focuses light behind the retina, so they need a converging (plus) lens to bring the focal point forward. Young hyperopes may compensate with accommodation, meaning their uncorrected vision can appear normal despite having a plus prescription. This is why some children are prescribed plus lenses even though they seem to see fine -- the glasses reduce the effort their eyes are making to compensate.
Plano and Sphere Notation
When the sphere is zero (no correction needed for that component), it may be written as "Plano," "Pl," or "0.00." When there is no cylinder, the prescription may write "Sph" or "DS" (diopter sphere) in the cylinder column, meaning the entire correction is spherical -- no astigmatism correction is needed.
If the eye were perfectly spherical like a basketball, only sphere power would be needed. But most eyes are slightly oval -- more like a football -- with different curvatures in different meridians. This is astigmatism, and it requires a cylinder correction that adds power in one specific direction without affecting the perpendicular direction.
The cylinder value tells you how much astigmatism correction is needed. The axis (a number from 1 to 180 degrees) tells you the orientation of that correction. Cylinder and axis always go together -- you cannot have one without the other. If the cylinder is blank, the patient has no significant astigmatism.
Most optometric prescriptions write cylinder in minus form (e.g., -1.25 x 090). Ophthalmologists traditionally use plus cylinder form (e.g., +1.25 x 180 for the equivalent correction). Both describe the same lens -- they are just different notations. You will need to know how to transpose between the two forms for the CPOA exam. The axis is always written after the cylinder, separated by an "x" symbol.
Around age 40-45, the crystalline lens inside the eye loses its flexibility and can no longer change shape enough for comfortable near focus. This is presbyopia, and it affects virtually everyone. The add power is the extra plus power added to the distance prescription to provide clear near vision.
Add power is always positive and typically ranges from +0.75 D (early presbyopia) to +3.00 D or more (advanced presbyopia). It is usually the same for both eyes. The add power appears on the prescription as a single number applied to both OD and OS, and it determines the type of multifocal lens needed: bifocal, trifocal, or progressive.
To calculate the near prescription, add the add power algebraically to the sphere. For example, if the distance Rx is -2.00 with an add of +2.00, the near Rx is 0.00 (plano) -- the patient needs no correction at near. If the distance Rx is +1.00 with an add of +2.50, the near Rx is +3.50.
Common Exam Question
The CPO and CPOA exams frequently ask you to calculate the near prescription from a distance Rx and add power. Practice this: distance sphere + add = near sphere. The cylinder and axis remain unchanged.
Not every prescription includes prism -- it is only prescribed when a patient has a binocular vision issue that causes the eyes to not align properly. Prism displaces the image so the eyes do not have to work as hard to fuse images from both eyes into one.
Prism is measured in prism diopters (represented by a triangle symbol or the abbreviation PD) and always has a base direction: BU (base up), BD (base down), BI (base in, toward the nose), or BO (base out, toward the ear). The base direction tells you which way the thickest part of the prism is oriented, which determines which direction the image is displaced.
On a prescription, prism might appear as: "2 BO OD, 2 BI OS" or "3 BU OD." The amount and direction must be entered precisely when ordering lenses, as even small errors in prism can cause diplopia (double vision) or headaches.
Pupillary distance is the measurement in millimeters between the centers of the patient's two pupils. This measurement determines where the optical centers of the lenses are placed within the frame. If the optical centers are not aligned with the patient's pupils, the patient experiences unwanted prismatic effect, which can cause eyestrain, headaches, or even diplopia in extreme cases.
PD can be measured as a single binocular number (the total distance between both pupils, e.g., 64 mm) or as monocular measurements (the distance from each pupil to the center of the nose, e.g., OD 32 / OS 32). Monocular PDs are more precise because most people have slight facial asymmetry -- their nose is not perfectly centered between their eyes. Monocular PDs are required for progressive lenses, where precise alignment is critical.
| Abbreviation | Meaning |
|---|---|
| OD | Oculus dexter -- right eye (always listed first) |
| OS | Oculus sinister -- left eye |
| OU | Oculus uterque -- both eyes |
| Sph / DS | Sphere only -- no cylinder correction needed |
| Pl / Plano | Zero power (no correction for that component) |
| DV / NV | Distance vision / Near vision |
| SVD / SVN | Single vision distance / Single vision near |
| BIF / TRI / PAL | Bifocal / Trifocal / Progressive addition lens |
| Bal | Balance lens -- non-prescription lens to equalize weight |
| BU / BD / BI / BO | Prism base direction: up, down, in (nasal), out (temporal) |
OD: -2.50 Sph
OS: -3.00 Sph
PD: 63
This patient is myopic (nearsighted) in both eyes. No cylinder means no significant astigmatism. No add means they are likely under 40 or do not need reading correction. The left eye is slightly more myopic than the right. Single vision distance lenses would be ordered.
OD: +1.75 -0.75 x 095
OS: +2.00 -1.25 x 080
Add: +2.25 OU
PD: 31/31
This patient is hyperopic with astigmatism in both eyes and has presbyopia requiring a +2.25 add (moderate presbyopia, likely age 50-55). The monocular PD (31/31) is provided, essential for ordering progressive lenses. The left eye has more astigmatism (-1.25) than the right (-0.75). Bifocals, trifocals, or progressives would be appropriate depending on the patient's needs.
OD: -7.25 -1.50 x 175, 2 BU
OS: -6.75 -1.00 x 010, 2 BD
Add: +2.50 OU
PD: 32.5/31.5
This is a complex prescription. High myopia in both eyes, astigmatism in both (more in the right), presbyopia, and vertical prism (2 BU in the right eye, 2 BD in the left). The vertical prism suggests a hypertropia or vertical imbalance. Asymmetric monocular PDs (32.5/31.5) indicate slight facial asymmetry. This patient would benefit from high-index lens material due to the strong prescription and would need progressive lenses with carefully ground prism.
Entering the right eye prescription into the left eye field is one of the most common and most consequential errors. Always double-check that OD is first and goes with the right lens. Some handwritten prescriptions are unclear -- if in doubt, call the prescribing doctor.
Confusing plus and minus cylinder, or failing to transpose when needed, changes the entire prescription. If the lab expects minus cylinder and you enter plus cylinder (or vice versa), the patient will get the wrong lenses. Know which form your lab uses and transpose if the Rx is written in the other form.
Axis 180 and axis 090 are common values that look similar when written quickly. Entering 180 when the prescription says 080 gives the patient the wrong correction entirely. If the axis is off by even 10-15 degrees, the patient will notice reduced clarity. Always verify axis carefully.
The add power usually applies to both eyes even though it may only be written once on the prescription. Entering it for only one eye results in one lens with the correct reading zone and one without. This is a surprisingly common entry error, especially for newer staff.
Deep dive into what each prescription component corrects and how they work together.
Learn to transpose between plus and minus cylinder forms.
How to translate technical Rx language into patient-friendly explanations.
Browse all CPO and CPOA study topics in one place.
OD stands for oculus dexter (right eye) and OS stands for oculus sinister (left eye). These Latin abbreviations are standard across all eye care professions. OD is always listed first on a prescription. OU (oculus uterque) means both eyes and is used when a single value applies to both, such as an add power for reading. Some newer prescriptions may use RE (right eye) and LE (left eye) instead.
Look at the sphere (Sph) value. A minus sign (-) before the sphere indicates myopia (nearsightedness) — the patient sees near objects clearly but distant objects blurry. A plus sign (+) before the sphere indicates hyperopia (farsightedness) — the patient may struggle with close-up work and, at higher amounts, distance as well. The larger the number, the stronger the correction needed. For example, -5.00 is significantly more myopic than -1.50.
The add (addition) power is extra plus power added to the distance prescription for near (reading) vision. It compensates for presbyopia, the age-related loss of focusing ability that typically begins around age 40-45. Add power is always a positive number, usually ranging from +0.75 to +3.00 diopters. It is the same for both eyes in most cases. The add is used to order bifocals, trifocals, or progressive lenses.
Prism displaces light to compensate for eye alignment issues such as strabismus (eye turn), convergence insufficiency, or diplopia (double vision). Prism is measured in prism diopters and has a base direction (BU = base up, BD = base down, BI = base in toward the nose, BO = base out toward the ear). Not every prescription includes prism — it is only prescribed when the patient has a binocular vision problem that cannot be managed with lenses or therapy alone.
PD (pupillary distance) is the distance between the centers of the two pupils, measured in millimeters. It determines where the optical centers of the lenses should be positioned in the frame. Incorrect PD causes the patient to look through the wrong part of the lens, inducing unwanted prismatic effect. PD may be recorded as a single binocular measurement (e.g., 63 mm) or as monocular measurements for each eye (e.g., OD 31.5 / OS 31.5). Monocular PDs are more accurate and are essential for progressive lenses.
Common abbreviations include: DV (distance vision), NV (near vision), SVD (single vision distance), SVN (single vision near), BIF (bifocal), TRI (trifocal), PAL or PROG (progressive addition lens), Sph (sphere — also used to indicate no cylinder correction), Bal (balance lens — a non-prescription lens for an eye that cannot be corrected), and Plano or Pl (zero power, no correction needed for that component).