We often take high-index as a magic pill, and many retailers would encourage you fork out extra for upgrading to thinner lenses. But, like the post on blue lenses, we shall now explore "does it actually work?" as well take a quick look at lens indexes (or indices).
TLDR Summary: There are many factors that come into play when it comes to lens thickness and indexes - it isn't just about trying to go for the highest index possible!
Lens Materials and Indexes / Indices
We've come a long way in terms of lens materials - many many years ago we would just be fitting glass lenses, then came along CR-39, also know as hard resin lenses, or to the consumer, just "plastic lenses".
Then thinner materials were developed, giving rise to the high-index lenses of today, together with other speciality materials like Polycarbonate and Trivex.
What is High Index?
Index basically just refers to how dense the material is, hence a higher density or higher index would mean the lenses can be done thinner while still giving the desired power (refractive correction).
There isn't a rule on what can be defined as 'high-index' lenses, since anything of a higher index than the traditional CR-39 lenses (index 1.499 which usually gets round up to 1.50) is, well, technically 'high-index'. However for this article we regard high-index as 1.60 and above.
The Real Numbers - how much thinner?
Let's take a case of moderate myopia of -6.00 (600 degrees). On an average-sized frame with a rim diameter of 52mm and bridge size of 17mm (that's the numbers you'll see printed on some spectacles, seperated by a box like this: 52□17), and with the distance between the centre of the eyes being an average 62mm,
|Lens Type and Index||Thickness|
Going for a higher index does make the lenses thinner, however do you realise that the difference is less significant when we go up the really really high-index options? There is quite a difference when comparing 1.50 versus 1.60 - a whopping 1.6mm difference. However, between 1.67 and 1.74, the difference is just 0.4mm.
Considering that plastic frames are all the rage now, and plastic frames tend to be thicker than metal frames, that's good news - some of the lens thickness is hidden in the rim of the frame!
Let us take a look at the following side-view diagram :
If we were to put the lenses into a plastic frame that is 4.5mm thick at the rims, the amout of lens coming out at the the back would be 3.2mm for 1.50 index versus 1.6mm for a 1.60 index.
That is quite some difference - we halved the amount of lens thickness that can be seen from the side just by going from 1.50 to 1.60. A thickness of 1.6mm that can be seen - that should be quite acceptable by most wearers' standards!
Of course, we could go for an even higher index of 1.67, which would mean the thickness is further reduced to 1.1mm (not that significant a difference from 1.6mm!) ; or if we would really want no lens thickness to be seen at all - that would mean using 1.90 glass, which is usually a much much more expensive option. So the question now really is - do you think it is worth your money for that amount of difference in thickness? But wait, read on...
It isn't just about the Index
We know that the higher the myopia (and astigmatism), the thicker the lenses. But did you notice that I've also included the frame size and distance between the centre of the eyes?
These factors also affect how thick the lenses will be at the sides! And while we cannot adjust how far apart our eyes are, we can sometimes choose a smaller frame which will make the lenses thinner. Even the frame shape can also affect how thick the lenses look!
The Abbe Number and Light Transmission
There is actually one downside to high-index lenses. The higher the index, usually the lower the Abbe Number.
A lower Abbe number means that there is more chromatic abberations - which basically means you'll see fringes of colour or ghost images around the edges of objects. Why this happens is because the different colours of light refract differently through the lens, and eventually falls at different points on the back of your eye.
The amount of light transmitted through a higher index lens is also lesser than a lower index lens, although that is usually mitigated by using a good anti-reflection coating, which is why getting a pair of high index lenses from one of the major lens brands is a wise choice.
|An example of a "plus power" lens - thin at the centre and thick at the sides.|
A Trick for High Plus Powers
You may have realised that plus powers lenses, used to correct far-sightedness and presbyopia in people who do not wear glasses for distant vision, are thick at the centre and thin at the sides.
For those with significant amounts of power in the plus direction, like +5.00 (yes it happens when you have long-sightedness AND presbyopia...), the lenses can end up being quite thick in the centre (and very heavy too!).
Using high-index lenses do also play a part in making these lenses thinner, but there's a lesser known trick - use a smaller frame, and request for smaller lenses to be used.
The logic behind this is simple - when a large frame is used, the lenses have to be large in order to fill up the entire space (duh, who wants a gap in the glasses?). But larger lenses also cause the lenses to be thicker in the centre, as seen in the picture below:
Therefore, by using a smaller frame, a smaller lens can be used, giving an reduction in lens thickness!
So to conclude, it is all about finding the right balance - the frame, the lens index, the vision we expect, and the cost. Are high index lenses always better? Maybe. Maybe not.
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