Photo: All About Vision
If your eyecare professional advises high-index lens materials for your eyeglass prescription, it is usually due to the strength of your prescription.
High-index lenses bend the light as it passes through at a different speed than standard index lenses. This enables the lenses to be made thinner than traditional lens material. There are several different levels of high-index materials. Which one is best for your prescription depends on the power of your lenses and the size of the frame the lens is to be inserted into.
Polycarbonate is a very common mid-index lens which is used for children for safety concerns (it is extremely impact resistant) and for low to mid-level prescriptions for durability and light-weight features and for lenses being put into large frames.
High-index lenses are commonly used for higher powered prescriptions to keep the thickness of the lenses down to a minimum. They are also usually used in rim free or partially rimmed frames due to the tensile strength of the material, which is less likely to chip or crack under stress.
If you have a high-powered Rx, you probably have experienced some unduly thick and heavy lenses in the past. Your eyecare professional can steer you to the best material for minimum thickness and weight in your prescription. Generally speaking, the higher the lens index, the higher the cost. However it is an expense that is well worth it both cosmetically and for the comfort of wear.