The thickness of your lens is indicated by a lens index. The higher the index, the thinner the lens is.High index lenses are suitable for higher grade power, while low index lenses are suitable for lower grade power.If your total power is below 3, choose 1.56 Standard lens.If your total power is between 4 to 6, choose 1.61 Super Thin lens.If your total power is between 7 to 10, choose 1.67 Ultra Thin lens. *Note. The default lens index is 1.56. Additional charges may apply for high-grade, thinner index lens and complex prescription(astigmatism /myopia / progressive /multifocal), please email our prescription team at prescription@visioneerbrand.com before placing an order if you have higher and complex prescription.