If you have a high negative prescription, you’ll be familiar with thin lens options which are made from high index materials. The higher the index number, the thinner your lenses will be. These materials essentially have an ability to bend light into your eyes at a higher angle which in turn corrects your sight. There are 2 main material options; glass and plastic.
You might have also heard of aspheric and double aspheric lens options which can further reduce the bulkiness in your lenses, with added bonuses such as reducing the bug-eye effect (the distortion towards the edge) and generally giving you bigger fields of vision. Let us explain...
Glass - up to 1.9 high index
Yes, actual glass lenses. These can be quite pricey, and go all the way up to 1.9 high index. However, they’re not widely recommended for 2 main reasons - safety, and weight. Like all glass, it can smash if for example you get hit by a football, or fall over into something. The glass also adds weight, which can affect comfort, and nose slide.
Plastic - up to 1.76 high index
Plastic lenses are the go-to material that you’ll usually get offered. The benefits of higher index plastic lenses are that they can be made to be thinner and lighter.
A standard optician will usually offer 1.5 - 1.74 options depending on your prescription. A 1.74 lens is going to turn out around 50% thinner than a 1.5 comparatively.
A more specialist lens is also on the market by a Japanese company called Tokai Optical which is 1.76 or ‘the thinnest lens in the world’ which is available through some opticians.
Aspheric and double aspheric
Aspheric lens options can further reduce thickness by creating a more complex lens surface which isn't completely round (i.e. spherical like a globe). Flatter curves are created which gradually change over the lens surface. This can be done on one side of the lens (Aspheric), or both sides of the lens (Double Aspheric) - the latter double option being the thinnest and lightest. This is a technology that can be combined with a high index lens option to get the sleekest result.
Edging is another add on technique used to the reduce the thickness at the edge of your lenses. In simple terms, the edge of your lens is cut off, being sure not to interfere with your field of vision.
Bespoke surfaced lens options
While you're here, it makes sense to mention another lens choice - surfaced or 'digital' lenses. These lenses are made to order and totally bespoke to your prescription, your fitting measurements and your glasses frame, including the base curve. Digital technology and lens design software drives the machinery that makes them and grinds the design into a lens blank. The benefits include a more accurate lens so you get better vision and wider field of view, which is more relevant the higher your prescription is.