Eyeglass Prescription Converter
Preserve both principal powers and make every distance assumption explicit.
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- Free preview: field labels with converted powers and complete reports hidden.
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Educational optical arithmetic only. This is not a prescription, contact-lens fitting or visual-acuity conversion.
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For general information only, not medical advice. Consult a qualified health professional for advice about your situation.
How this tool is checked
- Sources
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This guide cites one source:
- Tests
- 1 automated test checks its results.
- Found an error?
- Report a problem or write to support@mysmartcopilot.com, with what you entered and the result you expected.
About the Eyeglass Prescription Converter
Inspect the arithmetic behind a supplied glasses prescription. The converter transposes plus/minus cylinder notation, calculates spherical equivalent, compensates both principal meridians for an explicit vertex-distance change, and applies a supplied near ADD or an explicitly chosen intermediate fraction. It is an educational explanation, not a prescription or a contact-lens fitting.
How to use it
- Enter the supplied sphere, cylinder and axis. ADD is optional in practice; enter zero when no addition is supplied.
- Choose both source and target vertex distances in millimetres. The model converts their signed difference to metres.
- Enter an explicit intermediate ADD fraction. Review both principal powers and any singularity before exporting the current report.
Fields and arithmetic
SPH is sphere, CYL is cylinder, AXIS is its orientation and ADD is the supplied near addition. Transposition uses S′=S+C, C′=−C and a 90-degree axis rotation. Spherical equivalent is S+C/2. Vertex compensation uses Ftarget=Fsource/(1−dFsource), where d is the signed source-minus-target distance in metres. The OpenStax optical-power reference defines optical power as reciprocal focal length in metres. The signed-distance equation here is a paraxial dimensional derivation applied separately to each principal meridian; it does not establish contact-lens fitting or clinical suitability.
Limitations
- A contact lens needs its own fitting and prescription; refractive arithmetic supplies no base curve, diameter, lens material or clinical suitability.
- Prescription powers cannot be converted to a 20/20-style acuity result.
- Prism describes a separate optical displacement and its base direction. PD describes pupil separation for fitting; neither is inferred from sphere/cylinder.
- No ISO/ANSI dispensing compliance or individualized recommendation is inferred. Rounding and lens design remain professional decisions.
Privacy
On this device: nothing is sent
Prescription inputs stay in this page. No patient details are uploaded or stored.
Frequently asked questions
What do I get without a pass?
Without a pass, Eyeglass Prescription Converter shows field labels with converted powers and complete reports hidden. Until you unlock it, the result can’t be downloaded or copied. A Pro, Premium or Ultimate pass, a one-time payment that never renews, unlocks the full result. The pricing page lists the passes and their prices.
What does cylinder transposition change?
It adds cylinder to sphere, reverses the cylinder sign and rotates the axis by 90 degrees. The two principal powers remain equivalent.
Why convert both meridians for vertex distance?
An astigmatic prescription has powers S and S+C. Each must be moved separately before reconstructing the cylinder; changing only sphere is incomplete.
Does full near ADD determine an intermediate power?
No universal intermediate power follows from the total ADD alone. The fraction is supplied explicitly and is not prescribed by this tool.