I am very nearsighted and wear ‘coke-bottle’ glasses. Can I get thinner lenses that look better?
Unfortunately the more nearsighted you are, the thicker your lenses need to be for you to see clearly. Luckily, there have been great innovations in spectacle lenses over the last 10-15 years, with new technologies being utilized to create thinner, stronger lenses. Long ago, spectacle lenses were only available in glass.
Then plastic become available and most people preferred the plastic due to the fact that they were much lighter, and also safer. If the glass lens broke, shards could damage the eye. This does not happen with plastic. The downside is that the plastic scratches easier than glass, but with proper care, this can be avoided (you have to stop cleaning your glasses with your shirt!). In the early days, only one plastic material was available, CR 39.
However, there have been lots of newer designs and materials created so you will have a greater selection to choose from than before. There are newer materials called hi-index (also called ultrathin) lenses and newer designs like aspheric designs that can make lenses thinner, lighter, and better looking than traditional materials.
How are the lenses made thinner?
As light hits a spectacle lens, it’s refracted (bent) so that it focuses on the retina. The amount of bending required depends on the prescription derived by the optometrist during an eye examination.
The higher your prescription, the more the light needs to be bent. The amount of the bending is achieved by the thickness of the lens and the material used. High index lenses are made of materials that are compressed, or denser, than regular plastic material, so that you get the same prescription in a thinner lens. ‘High index’ means that the lenses are constructed of a plastic or glass material that has a higher index of refraction. Currently the highest index plastic lens has an index of 1.71. Conventional plastic has an index of 1.49. There are several different materials with different indices of refraction that give corresponding varying degrees of thinness. For example, 1.71 is the highest index, but the most commonly used high index materials are 1.60 or 1.66, depending on the prescription. The higher the index, the more expensive the lens. With high index lenses, the less you get, the more you pay! You can get these thinner lenses in both single vision and bifocals. If one of your eyes needs a much stronger prescription than the other, you can get the ultra-thin material in that stronger side and regular plastic in the other so that the thicknesses match.
High index material allows the spectacle lens to be made thinner. The examples above show the same prescription lenses in both regular and hi-index lens materials.
The power of the lens is achieved by grinding curves into the front and back surfaces. Another way of making the lens thinner is to change the curvatures of these lens surfaces. Most lenses have a front and back surface curvature that is spherical, (round, like a ball). However, these lenses can be manufactured so that the curvature changes from the centre to the edge.
This is called aspheric design and allows the edge of the lenses to be thinner, while achieving the same power at the centre. This means that the lenses do not ‘bulge’ out of the frame edge as much as regular lenses. In this case, the lenses are made thinner by the design, not the material.
Lenses can be made 20-30% thinner than with regular plastic materials. These lenses are more expensive, but look much nicer. Your optometrist can advise you on the best type of lens material for optimal form and function of your new spectacles, depending on your prescription.
