Lens index, materials and coatings explained for online buyers

What refractive index actually changes

Refractive index describes how strongly a material bends light. A higher index bends light more, so the lens needs less curvature, and therefore less material, to reach the same power. For a minus (short-sighted) prescription that means thinner edges; for a plus prescription, a thinner centre. Nothing about the index changes the power itself: a −4.00 lens is −4.00 whether it is made in 1.50 or 1.74.

Two things limit the benefit. First, frame size: a small frame cuts the lens close to its centre, where it is thin anyway, so a high index buys less in a small frame than in a large one. Second, weight: higher-index plastics are denser, so a 1.74 lens is thinner than a 1.60 lens but not always lighter. Thinness and lightness are related, not identical.

The materials, side by side

Matching the index to your power

These are the working rules dispensing opticians use. They assume an average-sized frame; drop one step for a small frame, and consider one step up for a very large or very deep one.

• Up to ±2.00: 1.50 or 1.56. Thinness is not the problem at this power; spend on coatings instead.
• ±2.00 to ±4.00: 1.60. The best balance of thinness, clarity and price.
• ±4.00 to ±6.00: 1.67, in a frame on the smaller side.
• Above ±6.00: 1.74, a small frame and an aspheric design, which flattens the lens and reduces the magnified or minified look of the eyes.
• Rimless or drilled frames: Trivex, polycarbonate or 1.60 and above. Never 1.50.
• Children, sport and safety: polycarbonate or Trivex regardless of power, because impact resistance comes first.

Cylinder counts too. A −3.00 −2.00 lens is −5.00 along one meridian, so choose the index for the strongest combined power, not the sphere alone.

Abbe value: the hidden trade-off

As index rises, the Abbe value falls. A low Abbe value means the material splits colours more, so you may see faint colour fringes around high-contrast edges when you look through the outer part of the lens. Most people adapt within days, a well-centred lens with a good anti-reflective coating minimises it, and a 1.60 lens keeps clarity close to standard plastic.

Coatings, layer by layer

Hard coat

A clear scratch-resistant layer. Essential on polycarbonate and all high-index lenses, which are softer than standard plastic. Resists scratches; nothing makes a lens scratch-proof.

Anti-reflective (AR)

Multiple thin layers that cut reflections from both surfaces. Clearer night driving, fewer ghost images on screens, and your eyes are visible to others. Reflections increase with index, so AR is practically mandatory from 1.60 upwards. The faint green or blue residual colour is normal.

Hydrophobic / oleophobic top coat

Makes water bead off and fingerprints wipe away. Usually bundled with better AR packages and worth having in a humid, dusty climate.

UV protection

Polycarbonate, Trivex and high-index materials block UV by nature. Standard 1.50 plastic needs a UV treatment; most laboratories include it, but check the specification says UV400.

Blue-light filter

Reduces part of the blue-violet band. Independent reviews have found little evidence that it reduces eye strain or improves sleep. It is harmless and some people like it; treat it as optional, not as protection you are missing.

Tints, mirrors and light-reactive lenses

Sunglass tints and mirror finishes are cosmetic and glare choices. Light-reactive (photochromic) lenses darken outdoors and are a lens type in their own right, with their own pros and cons in hot climates.

What a sensible package looks like

Shops sell lenses in tiers, and the tier names tell you nothing. Judge a package by its contents. For a low power: 1.50 or 1.56, hard coat, AR and UV400. For a mid power: 1.60 with AR and a hydrophobic top coat. For a strong power: 1.67 or 1.74 with a premium AR, where the coating quality is easier to notice than the index. Branded lenses from the large manufacturers cost more than laboratory house brands and typically bring better AR durability, tighter manufacturing tolerances and a written warranty; whether that is worth it depends on how long you keep your glasses and how hard you are on them.

Ask what each tier contains before comparing prices. Opticians in the UAE usually publish their tiers; Optic One, for example, lists its prescription lens options online, which makes a useful reference for what each level should include even if you end up buying elsewhere.

Caring for coated lenses in heat and dust