The 1.56 refractive index of 1.56 lenses refers to the refractive index, the higher the refractive index the thinner the lenses will be at the same degree, 1.56 is a relatively low refractive index number. The 1.56 refractive index lenses are more suitable for 100-200 degrees, or more than 300 degrees higher, which should not be a big problem. If the degree is higher, the refractive index is lower, the lens will be thicker and heavier, not suitable for children or girls. Commonly used is 1.67, relatively high cost performance, and not very thick, the same price is also more affordable, if you do not consider the price, of course, the higher the refractive index, the lighter and thinner the lens more beautiful.