1.67 vs 1.74 High Index Lenses: Is the Price Difference Worth It for High Minus Prescriptions?

In ophthalmic optics, the edge thickness ($t_e$) of a minus power lens is governed by the Sagitta Formula and Snell's refraction law:

$$s = r - \sqrt{r^2 - y^2} \quad \text{where } r = \frac{n - 1}{F} \implies \mathbf{\text{Higher refractive index } n \implies \text{Flatter curve } r \implies \text{Thinner edge } s}$$

Compared to standard plastic (1.50 CR-39):

• 1.60 High Index: $\sim 20-25\%$ thinner than standard plastic.
• 1.67 High Index: $\sim 35-40\%$ thinner than standard plastic.
• 1.74 Ultra-High Index: $\sim 45-50\%$ thinner than standard plastic.

However, when comparing 1.74 directly to 1.67, the thickness difference is only about $10\%$ to $15\%$. On a $-6.00\ \text{D}$ lens in a compact frame, that translates to a difference of just $0.6\text{ to } 0.9\ \text{millimeters}$!

As the refractive index climbs, optical clarity slightly decreases due to lower Abbe value ($V_d$):

$$\begin{cases} \mathbf{\text{1.67 MR-7 Resin:}} & n = 1.67 \implies \mathbf{V_d = 32} \\ \mathbf{\text{1.74 Mitsui MR-174:}} & n = 1.74 \implies \mathbf{V_d = 33} \end{cases}$$

Both materials have very similar Abbe values ($\approx 32-33$), meaning both require high-quality anti-reflective coatings (like BluePro HMC) to prevent chromatic dispersion and ghost reflections.

• Powers up to -4.00 D: Stick to 1.60 High Index (best clarity, durability, and cost).
• Powers from -4.25 D to -6.50 D: Choose 1.67 High Index. It hits the absolute sweet spot of thinness, lightness, and price. Upgrading to 1.74 in this range is rarely worth the extra cost.
• Powers of -7.00 D and Stronger: Choose 1.74 Ultra-High Index. At high minus powers ($-7.00\text{ to } -12.00\text{ D}$), even a $15\%$ reduction shaves off $1.5\text{ to } 2.5\ \text{mm}$ of edge bulk, dramatically improving cosmetic elegance and reducing weight on your nose.