All About Spectacle Lenses

This is the go-to article for all about spectacle lenses, which answers the most common doubts we have about optical lenses. Read on, learn more, and pay less. Got a question? Contact us!


Content:

Lens Index (and Thickness)

Anti-Reflective (AR) and Ultraviolet (UV) Coating

Violet-Blocking (HEV420)

Stock Range

Single Vision, Progressives, And Office Lenses

LENS INDEX (AND THICKNESS)


Probably the most important factor in our local context, we often ask for “high-index” lenses especially if we have higher ‘powers’ or prescription.


The common indices (or indexes) are 1.50, 1.56, 1.60, 1.67, and 1.74. Plastic lenses are usually available in these indices. Glass lenses, which we very seldom use these days, come in 1.523 (the legendary crown glass), 1.7, 1.8, and 1.9 (usually very expensive!).


You may also sometimes spot indices like 1.53 (Trivex) and 1.59 (Polycarbonate) – these are usually used for their higher impact-resistant, break-resistant, or higher tensile strength properties, such as in safety eye-wear. Sometimes, certain manufacturers use 1.55 instead of 1.56, or 1.61 instead of 1.60 – they are pretty much the same thing.


Advantages of high-index lenses


Generally, we regard index 1.60 and above as being high-index, but as there isn’t a set index number where lenses become “high-index”, it is always prudent to check as some sellers regard 1.56 as high-index.


High-index lenses have a higher density, or you can imagine it as having the material ‘compacted’ into a smaller area. This provides an obvious advantage – it is thinner. However, the greatest difference in thickness comes in higher prescriptions.


Disadvantages of high-index lenses


In high-index lenses, the Abbe value is often lower. This means poorer optics due to chromatic aberrations, or the spreading out of the different colours of light. Hence, it is important to consider and balance out the gains in reduced thickness versus optics.


High-index lenses often also cost more – and you may be surprised at how little difference it makes in thickness in certain circumstances.


Using mathematical formulas, we can calculate that for someone of -4.00 of myopia (400 degrees), with a typical interpupillary distance (distance between pupils, that centre part of your iris) of 62mm, wearing a typical frame diameter of 52mm and bridge size of 18mm, on an index 1.56, 1.60, 1.67, and 1.74 lens, the edge thickness of the lenses will be 4.9mm, 4.7mm, 4.3mm, and 4.0mm respectively.


There is less than a millimetre of difference – but probably costs quite a fair bit more, or a lot more for genuine 1.74 index lenses!


With the same parameters, at a much higher -12.00 of myopia (1200 degrees!), the edge thickness of an index 1.60, 1.67 and 1.74 lens will be 11.7mm, 10.5mm and 9.4mm respectively. There is a greater difference here.


Just remember, LOW POWER - HIGH INDEX - NO DIFFERENCE. If you're topping up for those amazing super-ultra-high-index-thinnest-lenses that aren't reasonably visibly thinner, then what exactly are you topping up; the salesperson's commission?


Intriguing? Visit us in store and find out more. We will probably go a little more in-depth in this on another blog post, as there are some situations where a significant thickness difference occurs.

ANTI-REFLECTIVE (AR) AND ULTRA-VIOLET (UV) COATING


Gone are the days where your optician tries to get you to pay more for these two. The AR and UV coating come as a standard these days in good optical shops.


The AR coating, as its name suggests, reduces the irritating reflections on the lenses. This means that more light can go through, increasing the light transmission through the lenses – something that you’ll definitely appreciate, especially at night.

The UV coating, as its name also suggests, reduces the amount of ultraviolet rays going through the lenses. We know that UV has harmful effects on the eye, such as increasing the risk of cataracts and possibly macular degeneration.


Hence, it is important that your lenses come with these two functions. We have it as a standard, even for our package and offer glasses.

VIOLET-BLOCKING (HEV420)

This interesting topic needs to start off with blue-light blocking lenses. Some time back, blue-light blocking lenses were all the rage – if you use the computer, mobile phone, or tablet, you need it or else some mysterious rays will kill off your eye cells!


Blue-Light Blocking Lenses


But… Does it really reduce eye strain or protect your eyes? Or is it just some marketing stuff?


Harvard Health has this to say: “blue light from electronic devices is not going to increase the risk of macular degeneration or harm any other part of the eye” [1]. In fact, a study that ran a double-blind test has shown little evidence that using such lenses prevent digital eye-strain [2]. Also, the American Academy of Ophthalmology does not recommend any special blue-light blocking eyewear for computer use [3].


[Read more about the blue-light marketing myth here.]


Violet-Light Blocking Lenses

This then brings us to the “new-generation blue-light blocking lenses”.


Violet light, also known as High Energy Visible 420nm (or HEV420 for short), has higher energy compared to the other colours in the visible light spectrum, as it it closer to ultraviolet (aha!), and can be harmful in larger amounts [4][5][6].

The biggest source of violet light (or blue light or light in general) is not your mobile phone (which is about 500 nits bright) or your computer screen, unless you live in a cave, but the sun (1600000000 nits) [7]. Which is quite common knowledge, really, and you don’t need to stare at the sun to find out.


So these lenses are great for general use! And unfortunately not a reason to be always on your mobile phone ;)


We might go more in-depth into this in another blog post, but that’s the summary for it for now.

[Update: We talk all about HEV420/Violet-Block Lenses here.]

STOCK RANGE


Ever got told that your prescription is “out of the range”? Maybe you have high astigmatism and end up paying more for your glasses?


The optical world (and the world…) is a matter of demand and supply. Lens manufacturers, suppliers, labs, and (some) optical shops are more inclined to keep lenses that see higher demand – the common prescriptions basically.


In general, optical shops tend to keep lenses with myopia up to -6.00 (600 degrees) and hyperopia/reading up to +4.00 (400 reading), with astigmatism up to -2.00 (200 degrees). Labs may stock a wider range, sometimes with myopia up to -10.00 (1000 degrees). Since most people fall within these ranges, it makes most sense to keep these in stock.


Therefore, when you fall outside this range, it’s a matter of either looking for a larger lab or supplier that keeps the higher prescription, or doing ‘made-to-order’ lenses. Consequentially, it takes more time and costs more.


It’s a fine balancing act – keeping a larger range means more wastage if those lenses are not ordered before they hit their shelf life, with the additional costs being placed into the other lenses (the common-prescription range), but can drastically reduce the time and cost for the consumer with the high prescription.


We approach this issue by using data, and dynamically adjusting our stocks and range to suit the demand ;)

SINGLE VISION, PROGRESSIVES, OFFICE LENSES


Typically, if you are under 40 years old, you would just walk into an optical shop, get your eyes tested, and do a pair of glasses. These come with single vision lenses – we correct your vision for far, and your eyes will adjust and focus automatically when viewing near objects (the technical term: accommodation). Simple as.


Then, about 40 years old, a problem surfaces – presbyopia, or old flower (老花), or old-age reading problem. This is when our eyes start to lose the focusing ability (reduced accommodation). When we correct your vision for far, you have problems adjusting the focus for near, thus making near objects blur, difficult, or tiring to view.


Here, you have two main options. Either two pairs of glasses, one for distance vision and one for near vision, or progressive glasses that come with both the far and near vision function in one pair of lenses. Both options have their pros and cons, depending on factors like your lifestyle, visual demands, and how well you think you can adapt to progressive lenses. This is something that you should talk to your optician about, in order to make the best decision, as there are also alternative options such as office lenses, reading glasses over contact lenses, and intermediate lenses.


Just a quick note: Progressive lenses are also known as multifocals, varifocals, and no-line bifocals, depending on where you are in the world.


Some lucky fellows with a myopia of about -2.00 (200 degrees) or so may find that removing their glasses will help them to see near, and so, are happy to do just that. Yay! This is because for that prescription, an object held at about 50cm away from the eyes is clear.

Office lenses

If you’ve tried progressives and found them a bit not right for office work, you’re not alone! Progressive wearers that are lucky enough (or unlucky enough, depending on how you want to see it) to spend most of their day on the computer may realise that they have to tilt their heads backwards in order to access the near area on their glasses in order to view the computer screen. This can eventually lead to discomfort, neck strain, and headaches.

Of course, manufacturers came up with a solution to this problem – Office Lenses. Sometimes also called Workspace lenses, these provide a huge intermediate zone (for that desktop monitor) and near zone (for the mobile phone and desk-paper-work if you haven’t gone fully digital). The distance zone is almost non-existant, although manufacturers often include a small area right at the top of the lenses for it, so that you can still see the time from the clock on the wall, or spot your boss coming to check on you.


To go a little further in customising such lenses, the intermediate zone can be adjusted; say you need to look at multiple screens about 1.5m away from you instead of just one screen about 60cm away.


Sounds cool? Do office lenses really work? From our experience, the adaptation is often very fast, probably because it’s just meant for use at the desk, and not for walking/driving, hence there isn’t the floating-effect like when wearing progressive lenses for the first time. Talk to your optical shop about this to see if it’s suitable for you.


Got a question or a topic you think we should include in there? Get in touch with us.




Information about Spectacle Lenses - Otago Optical Chinatown Optical Store



Sources:


[1] Harvard Health (2019) : Will blue light from electronic devices increase my risk of macular degeneration and blindness? , retrieved from https://www.health.harvard.edu/blog/will-blue-light-from-electronic-devices-increase-my-risk-of-macular-degeneration-and-blindness-2019040816365


[2] Rosenfield, M., Li R.T., Kirsch, N.T. (2020) : A double-blind test of blue-blocking filters on symptoms of digital eye strain, Work , retrieved from https://pubmed.ncbi.nlm.nih.gov/32007978/


[3] Vimont, C., Khurana, R., Hazanchuk, V. (2021) : Should You Be Worried About Blue Light?, AAO , retrieved from https://www.aao.org/eye-health/tips-prevention/should-you-be-worried-about-blue-light


[4] Grimm, C., Wenzel, A., Williams, T., Rol, P., Hafezi, F., Remé, C. (2001) : Rhodopsin-mediated blue-light damage to the rat retina: effect of photoreversal of bleaching, Investigative Ophthalmology and Visual Science , retrieved from https://pubmed.ncbi.nlm.nih.gov/11157889/


[5] Youn H.Y., Chou B.R,. Cullen A.P., Sivak J.G. (2009) : Effects of 400 nm, 420 nm, and 435.8 nm radiations on cultured human retinal pigment epithelial cells, Journal of Photochemistry and Photobiology , retrieved from https://pubmed.ncbi.nlm.nih.gov/19201202/ and https://www.sciencedirect.com/science/article/abs/pii/S1011134409000025


[6] Sapkota, R., Pardhan, S. (2016) : Eye Complications of Exposure to Ultraviolet and Blue-Violet Light, International Review of Ophthalmic Optics , retrieved from https://www.researchgate.net/publication/308844699_Eye_Complications_of_Exposure_to_Ultraviolet_and_Blue-Violet_Light


[7] Herman, J. (2010) : Giz Explains: Brightness, Gizmodo , retrieved from https://gizmodo.com/giz-explains-brightness-5649389