Anything over a 1.60 index can generally be described as “high index.” Lenses at 1.74 and beyond fall into the category of ultra high lenses. Those who have extremely high prescriptions (plus or minus 6.00 D or more) experience the greatest benefits from the reduced weight and thickness of high or ultra high index lenses.
So how high can high index lenses actually go?
1.90 is the highest index lens available. Since any high index lens at or above 1.80 are made of glass only, 1.90 lenses are, obviously, available only in glass and not in the plastic high index format. The primary benefit of 1.90 is the thinness of the material, as 1.90 lenses are as thin as it gets. The primary disadvantage is cost, as 1.90 ultra high index lenses are the more expensive than any other lens material currently available.
Weight is also a factor with 1.90 lenses. The glass they use is very dense and can become uncomfortably heavy if cut in a larger lens style. For this reason, consumers of 1.90 ultra high index lenses are generally advised to pick sturdy frames (the rimless style is not available for any glass high index lenses, let alone an index this high) and limit the physical lens size to cut down on material usage.
Color bleeding and prismatic or rainbow side effects are additional drawbacks of 1.90 lenses. They’re also limited in terms of color. 1.90s cannot be tinted and only come in clear.
With all these negative aspects, you may wonder what it is about 1.90 ultra high lenses that make them a feasible choice. To answer that question, look no further than that big advantage they have, thinness. Those with extreme prescriptions are often forced to endure the drawbacks that very thick glasses possess: distortion, an undersized or oversized look to your eyes, the “Coke bottle” or “bug eye” look. 1.90 ultra high lenses go a long way towards eliminating (or at least minimizing) these characteristics. And since they’re made of glass, they also enjoy the side benefits of increased durability and scratch resistance.
High index lenses are thinner, more powerful lenses. They're lightweight and stylish, but are mostly reserved for those with higher vision correction needs. While most prescriptions are compatible with more than one specific index, it's up to you to decide which one fits your personal needs!
The difference between your glasses' lens index can make a huge impact which is why learning about high index lenses’ meaning is important. Deciding which you should get depends on your prescription and your lifestyle. Thicker lenses are more affordable, but can only accommodate lower vision correction needs. Thinner lenses are lightweight (and more stylish!) and can accommodate MANY kinds of vision needs, including higher prescriptions. In terms of balancing price and quality, 1.59 index polycarbonate lenses accommodate most prescriptions and are extra durable.
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