Contact lens prescription vs glasses prescription: why they differ

Two prescriptions for one pair of eyes

A glasses prescription records the refraction, the lens powers that correct your vision at the spectacle plane. A contact lens prescription records the result of a fitting: powers adjusted for the eye’s surface, plus the physical shape of the lens that sits comfortably on your cornea and the specific product the practitioner fitted. They are separate documents, and in most countries, including the UAE, a shop should not dispense contact lenses from a glasses prescription alone.

Vertex distance: why strong powers change

Spectacle lenses sit about 12 to 14 mm in front of the cornea. Moving a lens closer to the eye changes its effective power. For short-sighted (minus) prescriptions the contact lens needs less minus than the glasses; for long-sighted (plus) prescriptions it needs more plus. Below about ±4.00 the difference is smaller than a 0.25 step and practitioners usually ignore it. Above that it grows quickly.

The arithmetic behind the table divides the spectacle power by one minus the vertex distance (in metres) multiplied by that power. It is simple, but the rounding, the cylinder and the brand’s available powers all need judgement, which is why the practitioner does it rather than the customer.

Base curve and diameter

Base curve is the curvature of the back of the lens, typically between 8.3 and 9.0 mm; diameter is the lens width, typically 13.8 to 14.5 mm. Together they decide how the lens sits. A lens that is too steep or too small grips the cornea, feels fine at first and then causes redness and blurred vision by evening. A lens that is too flat or too large slides, blurs with each blink and may fall out. The practitioner chooses them from your corneal measurements and then watches the lens move on your eye, which is the part no form can replace. Each brand offers only one or two base curves, so the values are not interchangeable between products.

Astigmatism and toric lenses

Soft toric lenses come in a limited range: cylinder powers in steps such as −0.75, −1.25, −1.75 and −2.25, and axes usually in 10-degree steps. Your glasses cylinder of −1.00 at 85 degrees has to be mapped to the nearest option the brand makes. For small cylinders of about −0.75 or less, practitioners often use a spherical lens with the cylinder folded into the sphere (the spherical equivalent: sphere plus half the cylinder), because comfort and stability can matter more than the last quarter dioptre. Toric lenses also rotate slightly on the eye, and the practitioner measures that rotation and compensates for it. None of this is visible on a glasses prescription, which is why torics in particular should never be ordered from one.

Multifocal lenses and the ADD translation

A glasses ADD of +1.50 does not transfer directly. Multifocal contact lens brands group additions into Low, Medium and High, with each brand drawing the lines in its own place, and the design often puts slightly different powers in the two eyes to balance distance and near. Some practitioners fit modified monovision instead, with one eye biased to distance and the other to near. The prescription that results is specific to the design that worked for you.

Why the brand is part of the prescription

Lens materials differ in oxygen transmission, water content, stiffness and surface treatment. A lens that suits dry, air-conditioned office eyes may not suit someone who wears lenses for sixteen hours or sleeps in them occasionally. The fitting establishes that one product works on your eye, so the prescription names that product and its replacement schedule, whether daily, fortnightly or monthly. Swapping to another brand is often possible, but it is a re-fit rather than a substitution. Contact lens prescriptions commonly expire after a year because the health of the cornea needs checking under lens wear, not because the power is expected to change.

Reordering online without mistakes

• Order from the box end: the label on your current pack shows brand, power, BC and DIA exactly as fitted.
• Match the eye. Many people have different powers in each eye, and right and left boxes look identical.
• Never convert a glasses power yourself for anything above ±4.00. If a glasses prescription is all you have, start with a tool such as Optic One’s contact lens power calculator to see the likely figure, then have a fitting before the first order.
• Count the supply: a 30-pack of dailies per eye is one month, a 6-pack of monthlies per eye is six months.
• Keep lenses and solution out of heat. A sealed blister left in a hot car will not be improved by it; store them indoors below room temperature extremes and check the expiry printed on each blister.