I work in community practice as an eye care specialist and I recommend spectacles to my patients. I’m never sure when to recommend mid- and high-index lenses. Do you have any tips?

Mid- and high-index lenses are useful for high-powered prescription lenses but it can be difficult to remember which index to use for which powers.

This is how I remember.

For prescriptions less than ± 2 (best vision sphere) I recommend a standard index of 1.56.

For prescriptions greater than ± 2 but not greater than ± 4 I recommend a mid-index of 1.59.

For prescriptions greater than ± 4 but not greater than ± 6 I recommend a high index of 1.61.

For prescriptions greater than ± 6 but not greater than ± 8 I recommend a high index of  1.67.

For prescriptions greater than ± 8 I recommend a high index of 1.74.

Perhaps keep a small card near your desk to remind you of these categories.

I advise my patients that mid- and high-index lenses will help make their lenses lighter, and thinner and allow them to consider a greater range of frames.