Different index lens covers different SPH power range.

1.61 High Index Lens covers the SPH range from +3.00 to -6.00 and CYL range from -0.25 to -2.00..

1.67 High Index Lens covers the range from -0.25 to -10.00, CYL range from -0.25 to -3.00.

Customers with SPH power greater than -3.00 are recommended to use 1.67 Index Lens as 1.67 Index Lens is much thinner and lighter as compared to 1.60 Index Lens.