Guide to purchasing prescription glasses: 1. Choosing between single vision and progressive: - Choose progressive when you need both distance and near vision and your prescription includes an ADD value. - Choose single vision if you only need correction for distance or near vision. 2. Choosing the refractive index of lenses (1.56/1.61/1.67/1.74): (Higher refractive index yields thinner, more comfortable lenses) - Sum of SPH and CYL less than 300 degrees: 1.56 refractive index is generally recommended. 1.61/1.67 are also suitable for a thinner feel. - Sum of SPH and CYL between 300 and 500 degrees: 1.61 is generally chosen. 1.67/1.74 are also suitable for thinner lenses. - Sum of SPH and CYL between 500 and 800 degrees: 1.67 is generally chosen. 1.74 lenses are also suitable.

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