Lens Index & Coatings Guide

1.74 is both the thinnest and the densest material on this list. It only wins on weight because the thinness reduction is large enough at high powers to outweigh that density. Trivex is the opposite case — genuinely low density and decent thinness, which is why it's the standard pick for kids and rimless frames rather than pure cosmetic thinness plays. On a low prescription, jumping straight to 1.74 can add cost without a meaningful weight benefit.

A real example at ±10.00 D. Each row changes only one thing and holds the other fixed:

Row 2 (diameter) saves more than row 1 (index) — but only for a big enough cut. Trimming 65mm to just 60mm saves only 1.1mm, less than the index upgrade. When clinically and cosmetically appropriate, combining a higher index with an optimized effective diameter can provide a substantially larger reduction than either change alone. All three percentages here are measured against the same original 65mm/1.56 edge thickness (≈9.43mm), so the combined row can be read directly as (original − final) ÷ original — which is why 24.3% and 29.1% happen to sum to 53.4% in this specific case. That's a property of sharing one baseline, not a general rule that percentage reductions add — a percentage measured against a different starting thickness would not combine this way. The full dataset behind these numbers follows below.

Blank diameter is the biggest lever after index, but final edge thickness on a minus lens (or center thickness on a plus lens) also depends on prescription power, refractive index, the center thickness specified, base curve, and whether the lens is surfaced as spherical or aspheric/freeform.

Effective diameter (ED): the diameter the frame's lens opening actually requires is usually smaller than the blank ordered from the lab. A round or oval frame shape lowers the effective diameter — and therefore the edge thickness — even without changing the blank size.

Under the paraxial model used in the reference table below, the 24.3% savings figure for switching 1.56 → 1.74 is diameter-independent — that's a mathematical property of the paraxial approximation, not a universal property of real lenses (the exact-geometry section further down shows it actually drifts a little with diameter). Either way, it's a share of a different (smaller) starting thickness at every diameter, not a sign the two indices converge to one thickness.

At a 50mm blank, 10.00 D, paraxial model: 1.56 ≈ 5.58mm sag (6.58mm actual, with the 1.0mm minimum) vs. 1.74 ≈ 4.22mm sag (5.22mm actual) — still about 1.36mm thinner in 1.74, the same 24.3% cut as at 65mm, just applied to a smaller starting point. (The exact spherical values at 50mm are slightly higher — 5.89mm and 4.35mm — see below.)

Diameter sets how thick each lens starts out; index then removes roughly the same percentage off whatever that starting point is. A smaller blank and a higher index stack on top of each other — they don't cancel out or converge.

See "Exact vs. Paraxial Sagitta" further down this page for the exact-geometry numbers and how much the percentage actually shifts.

Scaling note: since sag is proportional to power, this dataset scales roughly linearly with Rx. At +6.00 D expect ~60% of these values relative to +10.00 D; at +14.00 D expect ~140%.

Chromatic difference of power is approximated as TCA = P / V_d, where P is the prismatic power at a point off-center and V_d is the Abbe number. Below a V_d of about 36, off-center viewing starts to show visible blue/red fringing around high-contrast edges.

Higher-index materials also reflect more light per surface. By the Fresnel equation R = ((n − 1) / (n + 1))², a 1.74 lens reflects ≈7.3% per surface (>14% total across both surfaces), versus ≈4% per surface for CR-39. This is why a multi-layer Anti-Reflective coating is treated as required, not optional, on 1.67+ lenses — it restores transmission above 98% and removes internal glare ghosts.

The percentage is no longer perfectly diameter-independent. Under the exact formula, the index-only saving (1.56 → 1.74, at −10.00 D) works out to 26.1% at a 50mm blank and 27.7% at 65mm — not the flat 24.3% the paraxial model gives at every diameter. The gap is small but real, and it grows at larger apertures, where the paraxial assumption (h ≪ r) holds least well.

The absolute-mm story is unchanged. Exact sag differences are still larger at 65mm (2.88mm saved from index alone) than at 50mm (1.54mm) — the same direction as before, just with corrected magnitudes. And the central point of this whole page still stands: index and diameter reductions stack rather than cancel. Combining both (1.56@65mm → 1.74@51mm) saves 5.86mm exactly, versus 5.04mm paraxial — the same "roughly half the original edge thickness" conclusion, just with a slightly larger true number.

Practical takeaway: treat the paraxial percentages earlier on this page (24.3% / 29.1% / 53.4%) as a close, easy-to-explain reference for patient conversations. For a lab quote or a chairside promise, use the exact figures here, and remember that the true percentage saving is itself geometry-dependent — it shifts a couple of points across the diameter range, rather than being one fixed number.

Every calculation on this page is a geometrical model for comparing index and aperture, not a guaranteed finished-lens thickness. Actual thickness also depends on: front and back surface curves, aspheric or atoric design, base curve selection, effective diameter, optical center location and decentration, minimum center- and edge-thickness rules, frame shape and boxing dimensions, material-specific design parameters, manufacturing tolerances, sphere/cylinder and prism, pantoscopic angle and wrap, and freeform surface optimization.

Use the figures on this page to compare options directionally with patients and to sanity-check lab quotes — not as a substitute for the lab's actual thickness calculation on a specific order.

Worth explaining to a patient asking "why does AR cost extra if hardcoat is already scratch-resistant?" — they're different layers doing different jobs, not redundant options. UV treatment is applied either in the substrate itself (many high-index materials block UV natively) or as an added layer.