The quality of substrate determines the durability of the lens and the reliability of the coating. Good substrate clear and bright, long use time and not easy to yellow; And some lenses do not use for a long time on the yellow, or even coating off. Good lens without any scratches, scratches, hairy surface, pitting, lens oblique to meet the light observation, the finish is very high. There is no spot, stone, stripe, bubble, crack inside the lens, and the light is bright.
The higher the refractive index, the thinner the lens, and the higher the price.