Keep the signs and units
Read one eye at a time. SPH gives the spherical component; CYL and AXIS describe the amount and orientation of astigmatic correction. The unit D means dioptres, while the degree sign beside AXIS means an angle. ADD, if present, describes additional near power, and prism needs both an amount and base direction. PD is a positioning measurement in millimetres, not another power. A prescription is a set of related fields, so copying only the largest number loses important information.
SPH and CYL are lens powers in dioptres; AXIS specifies cylinder orientation in degrees. ADD records additional near power when prescribed. Copy every sign exactly rather than converting a prescription yourself.
Separate prescription and fitting
PD and fitting height help position lenses in the chosen frame. Ask your dispenser to explain whether PD is recorded for each eye or as a total. A glasses prescription is not a contact lens prescription.
A worked example
Teaching example: SPH −2.00 D, CYL −1.00 D, AXIS 180°. The spherical equivalent is −2.00 + (−1.00 ÷ 2) = −2.50 D. That arithmetic compresses the prescription for some descriptive uses; it does not preserve the cylinder and axis and must not replace the original order. Likewise, 180° is not more severe than 90°. Keep the original notation, because prescriptions can use different cylinder conventions that require a proper conversion to compare.
What the sources can tell us
AOP · Your prescription explained ↗
The Association of Optometrists explains SPH, CYL, axis, prism and near or intermediate additions as separate parts of a written prescription.
Scope and limits: This explains notation. It does not verify a copied prescription or decide which lens or frame is suitable for an individual.
Read one line without losing its meaning
Teaching example only, not for ordering: SPH −2.00 D, CYL −0.50 D, AXIS 180°. These invented values are not a prescription for the reader.
Keep each number with its label
SPH and CYL express power in dioptres; AXIS expresses orientation in degrees. The number 180 is not another power to add to −2.00.
Keep the eye and signs
Copy the right and left eye separately and preserve every sign. If a value or label is unclear, check the original record before using it.
List what is still missing
A clear reading of SPH, CYL and AXIS does not answer every fitting question. Ask which measurements the selected lens design and frame require.
CHECK YOUR UNDERSTANDINGCan the spherical equivalent replace SPH, CYL and AXIS when ordering?
No. It deliberately discards information needed to reproduce the full prescription.
Sources & reading notes
Page updated:
- Rodenstock · Lens passport ↗ Manufacturer information
Documents the maker’s specifications or positioning for a market; it does not establish independent superiority or personal suitability.
- International Myopia Institute · Defining and classifying myopia ↗ Research guidance
Check the report’s definitions and intended population before applying a conclusion.
- AOP · Your prescription explained ↗ Professional body guidance
Explains terminology and dispensing practice; it is not a head-to-head product trial.
- ABDO · Explaining spectacle prescriptions, part 1 ↗ Professional body guidance
Explains terminology and dispensing practice; it is not a head-to-head product trial.
- ABDO · Explaining spectacle prescriptions, part 2 ↗ Professional body guidance
Explains terminology and dispensing practice; it is not a head-to-head product trial.
What changed · Added a source-linked notation check and a labelled example separating power from axis.