When Do You Need High Index Lenses?
Determining whether or not you can benefit from the use of high index lenses is not an exact science.
Do I Need High Index Lenses?
But it general, the stronger your prescription, the thicker and heavier your lenses will need to be. If your prescription rises above a certain benchmark, you may want to consider high index lenses; if it exceeds that benchmark you’re even more likely to enjoy the benefits of high index lenses.
So what is that benchmark? To figure this out you need to have a basic understanding of the script used for determining prescriptions. Examine yours after your next eye exam. You’ll see two categories, labeled OS and OD, and possibly a third one, labeled OU. These headings derive from Latin and are simply abbreviations that identify your left eye, right eye, or both eyes. Here’s what they mean:
- OS (oculus sinister) – the left eye
- OD (oculus dextrus) – the right eye
- OU (oculus uterque) – both eyes
Under these headings are numbers that relate to your eyesight for each eye. Your optical lab will use these numbers to fashion your lenses. The numbers are a measurement of the corrective level each eye will require in units called diopters. The diopter number can be either negative or positive. A negative number (starting with a minus sign) indicates that you are nearsighted and a positive number (starting with a plus sign) indicates that you are farsighted.
The further either of these numbers gets from zero – which is considered the standard for perfect vision – the stronger your corrective lenses will need to be. A number of -1.50, for example, means that you are nearsighted by one and a half diopters, which is a moderate rating and requires relatively thin lenses…so standard, low-index plastic or glass lenses would likely work well for you.
Conversely, if your prescription reads, for example, -6.00, it means you are nearsighted by a full six diopters. That’s a stronger prescription and will require lenses that are fairly thick and heavy if fashioned from low-index plastic or glass. A switch to high index lenses in this case would be recommended.
Below are some diopter rating ranges as they correspond to high index material. Find the diopter number on your prescription, compare it to this chart, and you’ll find the lens material that best suits your prescription. Bear in mind, however, that this is only a “rule of thumb” chart to be used as a general guide. Several factors, including price, should weigh in your decision to use low index, mid index, or high index lenses.
- If your diopter number falls between 0 and +/- 2.00, the recommended index material is 1.50.
- If your diopter number falls between +/- 2.00 and +/- 4.00, the recommended index material is 1.56.
- If your diopter number falls between +/- 4.00 and +/- 6.00, the recommended index material is 1.61.
- If your diopter number falls between +/- 6.00 and +/- 9.00, the recommended index material is 1.67.
- If your diopter number is greater than +/- 9.00, the recommended index material is 1.74 or greater (depending on the severity of the prescription, you may need to 1.80 or 1.90 high index glass lenses).