First,you should consider the overall weight..Large lenses and thick frames add up to heavy glasses, something most people find uncomfortable. If you have a strong prescription, get the smallest diameter lens that looks good on you.Second,read the label carefully. You want 100 percent UV (ultraviolet light) protection from both UVA and UVB rays. Some lenses are labeled as UV400, which means the same thing.

Thanks a lot! Your suggestions are very useful to me as I prepare to buy a pair of presciption sunglasses.