A prescription of -3.25 is low-to-medium, and there shouldn't be any
restrictions. You can save money by not using high-index materials,
and there won't be that much difference in thickness. CR-39 has first-
rate optical properties, and is usually the least expensive option.
"Pure" CR-39 has an index of 1.5, sometimes stated as 1.49. Materials
with a different index my contain CR-39 mixed with other materials,
but they're not "really" CR-39.
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