Generally speaking, the higher the index of your lens, the more it will cause distortion. This is because higher index lenses are more dense than standard ones. The more dense it generally is. The lens's density is related to its Abbe number, or the amount of aberration it causes.High index glass has an especially low Abbe number, meaning this material causes the worst color aberration.Many people have no problem with the amount of color aberration they get from high index lenses. The people who notice it the most are those with very strong prescriptions and very high index lenses. Anybody with high index glass will notice at least some aberration.1.70 high index plastic has a good balance between thinness, light weight, scratch resistance, and aberration. This is the best choice for most people.