A word about Index Numbers
Index numbers are used to dictate how thin or thick your lenses will be. As index numbers increase, the lenses in question are designed thinner and lighter, being better suited for higher power prescriptions. Take these measurements into consideration when purchasing lenses.
Standard (Plastic)
Standard lenses use plastic as their base material. Plastic lenses are often cheaper lens options that work well with smaller prescriptions, but have a strong tendency to become scratched or smudged through daily wear and tear. This results in glasses which are affordable, but not very durable.
Plastic lenses normally use a standard or mid-index number from 1.50 to 1.54 for a thicker design. As such, they are ideal for people with minor prescriptions.
Better (Polycarbonate)
A step up from plastic, polycarbonate lenses are made of a material that is light, thin, highly resistant to damage caused by impacts, and capable of blocking UV light. Given their advantages over cheaper alternatives, polycarbonate lenses are a great choice for those looking for durable, long-lasting glasses.
Polycarbonate lenses keep their index number around 1.55 to 1.59 for a slightly thinner appearance.
Advanced (Thinner)
Lenses with even thinner polycarbonate materials make use of higher index numbers to achieve a lighter and more comfortable fit for people with heftier prescriptions.
Advanced polycarbonate lenses typically opt for a high index number in the range of 1.60 to 1.68, and are best used for prescriptions exceeding +/- 4.00 powers.
Premium (Extra thin for higher powers)
The lightest lenses you can usually find, these hi-tech polycarbonate lenses utilize very high index numbers to achieve the thinnest and lightest designs possible; perfect for people with very large refractive errors. Note that these lenses will err on being more expensive than other options.
Premium polycarbonate lenses keep their index numbers nice and high above the 1.69 mark.