It is a common misconception that prescriptions for glasses and contact lenses are identical. However, since the devices are distinct, they each need their own prescription. The reason lies in their placement: eyeglass lenses sit about 12 millimetres from your eyes, whereas contact lenses rest directly on the eye's surface. This distance variation can lead to differences in prescription strength.
Given that contact lenses require an exact fit to your eyes, their prescriptions include specific details not found in your glasses prescription.
The illustration below outlines the key differences between contact lens and glasses prescriptions:
Common Elements in Both Glasses and Contact Lens Prescriptions
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Sphere (SPH): This represents the amount of correction you need for short-sightedness (myopia) or long-sightedness (hyperopia). A negative value (Minus -) indicates myopia, while a positive value (Plus +) indicates hyperopia.
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Cylinder (CYL): This is measurement assesses astigmatism, a condition in which the cornea is more oblong than round, resulting in blurred vision. The value can be negative (-) or positive (+) and represents the amount of lens power needed to correct the astigmatism.
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Axis: If there's a CYL value, there will be an axis value. It represents the orientation or angle (measured in degrees, within 0-180°) at which the cylinder is incorporated into the contact lens to correct your astigmatism.
- Add (or Near Add): This value helps with close-up vision, especially as we age. For glasses, it can be listed twice: once for close-up reading and once for intermediate distances, such as looking at a computer.
It's worth mentioning that for those with a slight astigmatism, regular contact lenses might be just fine. Many don't see a difference without the special adjustments.
In glasses prescriptions, the ADD will always be expressed as a numeric (Plus +) value. However, when it comes to contact lenses it could be expressed as either a numeric value or a MAX ADD value (LOW, MID, HIGH) to indicate a ‘range’ for correction, rather than a specific numeric value. In the context of contact lenses, it might also be referred to as Extra Power or Reading Addition.
Elements Unique to Contact Lens Prescriptions
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Base Curve (BC): The base curve determines the curvature of the contact lens. It's essential for the lens to fit comfortably on the cornea.
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Diameter (DIA): This measures the size of the contact lens. It ensures the lens covers the cornea properly.
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Lens Brand or Material: Contact lens prescriptions often specify a particular brand or material, and this is usually the product for which you have completed a successful trial. This is because certain brands might be more suitable for an individual's eye conditions or needs.
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Replacement Schedule: Some contact lenses are daily disposables, while others can be worn for longer periods. The prescription will specify how often the lenses should be replaced.
- Other recommendations: this could include the use of eyedrops for helping to keep the eyes hydrated, or a particular brand of solution used to care for (reusable) contact lenses.
Elements Unique to Glasses Prescriptions
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Pupillary Distance (PD): This is the distance between the centres of the pupils in both eyes, measured in millimetres. It helps in positioning the optical centre of the lenses directly in front of the pupils.
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Back Vertex Distance (BVD): This indicates the distance from the back of the lenses of your glasses to the front surface of your eye.
- Prism: This is used to correct eye alignment issues. It's measured in prism dioptres (Δ) and can have a base direction (e.g., base up, base down, base in, or base out).
Converting a glasses prescription to a contact lens prescription isn't as simple as it may seem. Online conversion tools can't match the accuracy of a qualified optician, and our Lenstore Opticians cannot convert prescriptions without a specific examination to ensure the contacts are safe and appropriate for your eyes.
A contact lens prescription is issued only after a specific contact lens fitting has been conducted and once the optician is confident the lenses are suitable for your eyes, reducing the risk of complications associated with wearing them.
And lastly, it's worth noting that not everyone who wears glasses is a suitable candidate for contact lenses. Always consult your eye doctor before acquiring any optical devices.