Thinner, lighter high-index lenses are especially recommended if you have a strong eyeglass prescription for nearsightedness, farsightedness or astigmatism. Depending on the base curve and frame, we may use 1.67 Hi Index on corrections of -3.00 to +3.00 and 1.74 Hi Index when correction is stronger such as -8.00. A 1.67 high index lens is a worthwhile upgrade. You are likely to experience notable optical distortion without hi index. This will also reduce the overall weight of the lenses making the frames more comfortable. These lenses are an up-charge of $150 so if you do not have a strong prescription, you do not need to select hi index.