Double Convex and Double Concave High Index Lenses
High index lenses are intended to help make your lenses thinner, and double concave and double convex high index lenses are designed to make your lenses even thinner still. Most lenses are made using a “lens blank” with a set curvature on the front, which is generally either convex or flat.
When the prescription is generated into the lens, it happens on the back side, so the front stays convex. For prescription glasses for nearsightedness, this can be an issue because the center needs to be thinner than the outside edges of the lens, so you end up with excess lens thickness at the edges. A similar situation holds true for people with farsighted prescriptions, only it’s the center of the lens that has an excessive amount of thickness. Double convex high index lenses and double concave high index lenses remedy this situation:
- Double concave high index lenses are concave on both sides, so the lens is thin in the center and thickest at the edges. These are the thinnest lenses for high index prescriptions for people who are nearsighted (who need a – prescription).
- Double convex high index lenses are convex on both sides, so the lens is thickest at the center and thinnest at the edges. This is best for prescriptions for farsightedness (+ prescriptions) and makes the lens as thin as possible.
- Having the curvature on both sides of the lens, as with double concave and double convex high index lenses, makes the lenses more aesthetically pleasing by making them more symmetrical and creating less edge thickness that shows from the side of the frames.
- Double curvature also reduces the amount of distortion that the lenses cause from the outside. This means your lenses won’t give you as much of the “bug eye” or “tiny eyes” look.
- Double curvature also improves the optical clarity of the lens by improving the focus of the optical center, minimizing aberration and streaking caused by high index lenses.
Many optical labs which specialize in prescription lenses for high or specialty prescriptions do double convex or double concave high index lenses on a regular basis. These lenses are a great choice if you have a very strong prescription and are looking for the best-looking and clearest high index lenses possible. If your prescription is not very high, double curvature may not be worth the added charges most places associate with the service, but for very strong prescriptions, it’s perfect. If you are interested in learning more about high index prescription lenses, take a look at some of our other posts, and thanks for reading!