Ultra high index lenses are a good choice for people with prescriptions over +/-6.00 sphere. Ultra high index lenses are available in 1.67, 1.71, and 1.74; in general, the higher the index, the thinner the lens but if your prescription is at the lower end of the range, using a higher index will not result in a thinner frame. The higher the index, the denser the lens becomes which results in a lower visual acuity. Thus, you want to find the lowest index that will work well with your prescription. Ultra high index lenses all include inherent UV protection, and they have the added advantage that they have minimal distortion to the size of your eyes as seen by other people.