Eye Prescription Converter: Plus-Cylinder to Minus-Cylinder
Understanding your eye prescription is crucial for clear vision. Optometrists and ophthalmologists write prescriptions in a standardized format, but sometimes you might encounter different notations, most commonly Plus-Cylinder and Minus-Cylinder. This calculator helps you convert a prescription written in the Plus-Cylinder format to the Minus-Cylinder format, which is often used by opticians and lens manufacturers.
What Do the Terms Mean?
- SPH (Sphere): This indicates the main corrective power needed for nearsightedness (myopia, indicated by a minus sign) or farsightedness (hyperopia, indicated by a plus sign). It's measured in diopters (D).
- CYL (Cylinder): This value corrects for astigmatism, an imperfection in the curvature of your eye's cornea or lens. It's also measured in diopters. A higher number (regardless of sign) indicates more astigmatism.
- AXIS: This number, ranging from 1 to 180 degrees, indicates the orientation or angle of the astigmatism. It tells the lens maker where to place the cylindrical power.
- ADD (Add Power): This is the "reading addition" power, typically for multifocal or progressive lenses. It's always a positive number and is added to the sphere power for near vision correction, primarily for presbyopia.
- PD (Pupillary Distance): This is the distance in millimeters between the centers of your pupils. It's essential for correctly centering your lenses in your frames.
Why Convert?
Historically, different types of equipment and professional preferences led to the use of both Plus-Cylinder and Minus-Cylinder notations. While most modern prescriptions are written in Minus-Cylinder, some older equipment or specific practitioners might still use Plus-Cylinder. Knowing how to convert between them ensures you can accurately interpret any prescription you receive and communicate it effectively to your optician.
How the Conversion Works
The conversion from Plus-Cylinder to Minus-Cylinder (and vice-versa) follows specific optical rules:
- New Sphere: Add the original Sphere and Cylinder values together.
- New Cylinder: Change the sign of the original Cylinder value.
- New Axis: If the original Axis is less than or equal to 90 degrees, add 90 to it. If it's greater than 90 degrees, subtract 90 from it. If the cylinder is 0, the axis remains the same (or can be considered irrelevant).
The ADD power and Pupillary Distance (PD) typically remain unchanged during this specific conversion, as they are independent of the sphere/cylinder/axis notation.
Enter Your Plus-Cylinder Prescription Below:
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width: calc(100% – 100px); /* Adjust width for D/Degrees text */
padding: 10px;
margin-right: 5px;
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font-size: 16px;
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function calculatePrescriptionConversion() {
// Get input values for Right Eye (OD) Plus-Cylinder
var odSphPlus = parseFloat(document.getElementById('odSphPlus').value);
var odCylPlus = parseFloat(document.getElementById('odCylPlus').value);
var odAxisPlus = parseInt(document.getElementById('odAxisPlus').value);
// Get input values for Left Eye (OS) Plus-Cylinder
var osSphPlus = parseFloat(document.getElementById('osSphPlus').value);
var osCylPlus = parseFloat(document.getElementById('osCylPlus').value);
var osAxisPlus = parseInt(document.getElementById('osAxisPlus').value);
// Get additional parameters
var addPower = parseFloat(document.getElementById('addPower').value);
var pupillaryDistance = parseFloat(document.getElementById('pupillaryDistance').value);
// Validate inputs
if (isNaN(odSphPlus) || isNaN(odCylPlus) || isNaN(odAxisPlus) ||
isNaN(osSphPlus) || isNaN(osCylPlus) || isNaN(osAxisPlus) ||
isNaN(addPower) || isNaN(pupillaryDistance)) {
alert('Please enter valid numbers for all prescription fields.');
return;
}
// Validate Axis range
if (odAxisPlus 180 || osAxisPlus 180) {
alert('Axis values must be between 0 and 180 degrees.');
return;
}
// Validate Cylinder sign for Plus-Cylinder input
if (odCylPlus < 0 || osCylPlus < 0) {
alert('Cylinder values for Plus-Cylinder input must be positive or zero.');
return;
}
// — Right Eye (OD) Conversion —
var odSphMinus, odCylMinus, odAxisMinus;
// New Sphere (OD)
odSphMinus = odSphPlus + odCylPlus;
// New Cylinder (OD)
odCylMinus = -odCylPlus;
// New Axis (OD)
if (odCylPlus === 0) {
odAxisMinus = odAxisPlus; // Axis is irrelevant if cylinder is 0, but we'll keep the original for consistency
} else if (odAxisPlus <= 90) {
odAxisMinus = odAxisPlus + 90;
} else {
odAxisMinus = odAxisPlus – 90;
}
// — Left Eye (OS) Conversion —
var osSphMinus, osCylMinus, osAxisMinus;
// New Sphere (OS)
osSphMinus = osSphPlus + osCylPlus;
// New Cylinder (OS)
osCylMinus = -osCylPlus;
// New Axis (OS)
if (osCylPlus === 0) {
osAxisMinus = osAxisPlus; // Axis is irrelevant if cylinder is 0
} else if (osAxisPlus <= 90) {
osAxisMinus = osAxisPlus + 90;
} else {
osAxisMinus = osAxisPlus – 90;
}
// Display results
document.getElementById('odSphMinusResult').innerText = odSphMinus.toFixed(2);
document.getElementById('odCylMinusResult').innerText = odCylMinus.toFixed(2);
document.getElementById('odAxisMinusResult').innerText = odAxisMinus;
document.getElementById('osSphMinusResult').innerText = osSphMinus.toFixed(2);
document.getElementById('osCylMinusResult').innerText = osCylMinus.toFixed(2);
document.getElementById('osAxisMinusResult').innerText = osAxisMinus;
document.getElementById('addPowerResult').innerText = addPower.toFixed(2);
document.getElementById('pupillaryDistanceResult').innerText = pupillaryDistance.toFixed(1);
document.getElementById('result').style.display = 'block'; // Show the result section
}