The thickness of eyeglass lenses depends on several measurable parameters: prescription in diopters, refractive index of the material, dimensions of the frame, and position of the optical center. Comparing these variables allows for determining the best compromise between thinness, weight, and quality of vision, without relying solely on the highest index in the catalog.
Refractive index and lens thickness: what the data shows
The refractive index reflects a material’s ability to bend light. The higher it is, the less thickness the lens needs to achieve the same correction. Four indices cover almost the entire market for organic lenses.
| Refractive Index | Suitable Correction Type | Thickness Reduction (compared to standard index) | Peripheral Optical Comfort |
|---|---|---|---|
| 1.50 (standard, CR-39) | Low corrections | Reference | Very good |
| 1.60 | Moderate corrections | Significant | Good |
| 1.67 | Medium to strong corrections | Important | Fair |
| 1.74 | Strong corrections | Maximum | More limited |
This table summarizes a concrete trade-off: increasing the index reduces thickness but degrades peripheral vision. The 1.74 index produces the thinnest lens, but its Abbe number (which measures chromatic dispersion) is the lowest. The wearer then perceives more color fringes at the edges of the visual field.
To compare available options and estimate the resulting thickness before purchase, the services offered by Pharmidea detail the logic of manual calculation versus online simulators.
Diopter correction and frame choice: two variables as important as the index
Focusing the choice solely on the refractive index is a common mistake. The prescribed correction and the size of the frame influence the final thickness in sometimes more decisive ways.
Myopia and hyperopia: thickness is not located in the same place
A myopic lens is concave: thin in the center, thick at the edges. A hyperopic lens is convex: thick in the center, thin at the edges. This difference radically changes the strategy.
- For myopia, reducing the frame diameter directly decreases the thickness at the edge, sometimes as much as an index jump.
- For hyperopia, the frame diameter has less effect on the central thickness, and a higher index becomes the main lever.
- A precise centering (well-measured pupillary distance, correct mounting height) avoids decentering the thinnest point of the lens, which would limit the benefits of thinning.
Drilled, nylor, and full-rimmed frames: impact on thickness perception
Full-rimmed frames conceal the edge of the lens. On a drilled frame (without a rim), the edge remains visible. A 1.50 index lens on a drilled frame with moderate myopia correction will appear much thicker than an identical lens in a thick acetate full-rimmed frame.
The type of mounting alters the perception of thickness without changing the actual thickness. A wearer attached to drilled frames therefore has more reason to increase the index than a wearer of full frames.
Peripheral discomfort with very thin lenses: an underestimated compromise
Since 2024, professional optical magazines have reported an increase in complaints from wearers who have switched to high-index lenses. The recurring symptoms: distortions at the periphery, the impression of flattened objects at the edges of the visual field, while central acuity remains good.
This discomfort does not solely arise from chromatic dispersion (the famous Abbe number). It also results from flatter designs necessary to thin the lenses, which alter optical behavior away from the center. A wearer accustomed to 1.50 or 1.60 index lenses who switches directly to 1.74 may experience persistent discomfort for several weeks.
On the other hand, a wearer whose correction exceeds a certain threshold has little choice: staying with a low index produces a lens that is so heavy and thick that it slips down the nose and distorts the frame. The compromise then consists of aiming for the index just sufficient for acceptable weight and thickness, without always seeking the highest index available.
EN ISO 16321 standard and corrective protective lenses: a special case
For wearers of corrective glasses integrated into personal protective equipment, the regulatory framework is evolving. The EN ISO 16321 standard is gradually replacing older European references. It redefines classes of mechanical resistance, prompting manufacturers to favor materials more resistant to impacts.
In practice, this means protective lenses are often thicker or less thinned than city lenses. A wearer who uses corrective glasses at work (construction site, laboratory, workshop) must take this constraint into account: the very thin 1.74 index is not always compatible with the mechanical resistance requirements of the standard.
- Check if the chosen lenses bear the marking compliant with the EN ISO 16321 standard for the intended use.
- Prefer polycarbonate or Trivex for impact resistance, even if they are slightly thicker than high-index organic lenses.
- Confirm with the optician that the mounting (full-rimmed, nylor, drilled) is compatible with the standard’s requirements for the intended protection class.
The ideal thickness of an eyeglass lens cannot be reduced to an index checked on an order form. The prescription, the frame, the type of mounting, and daily use form a system. A medium index lens well-centered in a small diameter frame can produce a result that is thinner, lighter, and optically more comfortable than a 1.74 lens poorly suited to a large frame.