In this installment of our new back-to-basics series, we take a laser focus on lens materials and the pros and cons of each.

When discussing lens materials, whether it be with a client or an individual with the lens lab, the conversation is primarily focused on expectations. How thin or thick will the lenses be? Will the consumer understand the correlation and see value in upgrading for a higher-end lens material?

While sometimes the decision of what lens material to use is outside of our control as eyecare professionals, due to factors such as a patient’s budget or Rx range limitations, it is our ethical duty to explain the options, educate the patient, and, if need be, state our unwillingness to make a pair of glasses with an index that will result in a negative visual experience and appearance.

Before We Begin…

Before weighing the pros and cons of lens materials, it must be said that frame fit on a patient and the resulting decentration based on pupillary distance and optical center height actually have a big influence on the thickness and weight of the lenses—and can have more influence than the lens material choice. Ensuring that a patient’s frame selection is a good fit and compatible with their Rx will result in the most aesthetically desirable and visually sound pair of glasses.

Lens Materials

It is impossible to discuss lens materials without a review of the fundamentals of Abbe value and what it means in relation to lens materials. While Abbe value isn’t a topic that comes up in the lens material selection process with patients, knowing the effect the Abbe value can have on a patient’s visual experience can help troubleshoot tolerance issues should they arise.

All ophthalmic plastic lens materials have an associated Abbe value (a number between 30 and 58), which values the measurement of the lens material’s dispersion of light. A lens with a low Abbe value causes a higher dispersion and leads to unwanted chromatic aberration. Chromatic aberration is a distortion of the image due to the inability of the lens to focus all colors onto the same focal point, which a patient may describe as a rainbow halo effect emitting from lights. The higher the number, the better the visual optical performance. CR-39 plastic and 1.5 glass lenses have the highest Abbe value, although other characteristics of these materials typically make them a less desirable choice in lens index.

To what degree a lens index will affect the appearance of the finished pair of glasses can be difficult to articulate or display. Using tools such as lens thickness calculators can show a numerical example of anticipated lens thickness, based on Rx, PD, and frame size in different lens materials. Presenting the patient with the comparisons will help involve them in the lens index selection discussion and set a realistic expectation of the physical appearance of the completed glasses.