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Should You Buy CR-39 Lenses or Polycarbonate?

The Polycarbonate Sales Pitch

Thinner, lighter, 10x more shatterproof.

These and other virtues of polycarbonate lenses are extolled by your optician, when you are shopping for new glasses.

Odds are that you have no idea about lens materials. And why should you? Unfortunately, there are more than a few opticians who don't actually know a whole lot about lens materials. Others may, but still want the larger profit margin that comes from selling a poly lens. All this leaves you with less than perfect advice on what lenses to buy.

Here Is The Odd Fact: In The Case Of Lenses, Cheaper Might Actually Be Better.

Glasses started out with glass lenses. And glass is actually by far the best lens material, in terms of optical quality. You can buy a thousand dollar pair of poly lenses, and they won't have anywhere near the optical quality of a $15 glass lens.

Think I might be joking? Consider the lenses used in expensive cameras and in telescopes. Are those ever any type of polycarbonate or plastic? Never! The highest quality lens is always made of glass.

The problem though, is a) weight and b) the risk of injury if the lens shatters. This is how the road down alternative materials got its start.

Cheap, Simple, High Quality

Meet CR-39.
Meet CR-39.

Meet CR-39

CR-39 is the name of the monomer base that makes up the lens material in inexpensive plastic lenses.

Owned by PPG, this material revolutionized lens making. Half as heavy as glass, far less likely to shatter, and optical quality nearly as good as glass. CR-39 is heated and poured into optical quality glass moulds - adapting the qualities of glass very closely.

Most lenses in glasses today are made of CR-39 plastic, or very close copies (since the material is owned by PPG).

And Then Came Poly

Polycarbonate is still lighter, and can be made in higher index - meaning that a strong prescription lens can still be quite thin.

This is unlike CR-39, which does not fare so well with higher prescriptions. Since the refractive index is only 1.49, these types of lenses get very thick, if you need a strong correction. That's where polycarbonates begin to make sense.

Anything Below -2.00 or -.175 Diopters, Doesn't Require High Index Lenses.

Polycarbonate is also even more shatter resistant, making it ideal for activities that have a strong risk of high impacts to your lenses. Though in reality this is not much of an issue, since CR-39 itself is quite impact resistant. The sales pitch of "shatterproof" is just that - a pitch.

There are reasons, in addition to high prescriptions, that warrant poly lenses:

CR-39 Cracks Easily, When Drilled.

This only matters if you choose rimless glasses, where the mounting point of the lenses has to be drilled into the side of the lens. Polycarbonate takes well to this treatment, where CR-39 does not. So if you want a rimless design, you need to go with polycarbonate.

Polycarbonate: Thinner

Source

Optics As Priority

  1. Your eyesight tends to get worse, with high prescriptions.
  2. Your perception of your environment is affected by optical quality.
  3. Light is handled poorly by polycarbonate materials.
  4. Poly lenses are a significant optical quality compromise.

What Matters: Optics

Polycarbonate seems to be an easy sale, and optic shops will always steer your to poly, if you appear to have the extra cash to blow. And as we already discovered, if you have high myopia or want rimless glasses, this may indeed be the way to go. There is a fairly major downside to polycarbonate, however:

Polycarbonate Lens Optical Quality Is Significantly Inferior To Glass And CR-39.

And the difference is significant. What you gain in impact resistance and higher index, you pay for in how the lens transfers light into your eye. And much like a poor quality camera lens translates into a poor quality photograph, that poly lens transfers an image through your eye, into your brain, that is far, far less than your visual cortex expects.

Lens Material Affects Light Transmission

Source

Glass vs. CR-39 vs. Poly

Glass
CR-39
Polycarbonate
highest optical quality
good optical quality
poor optical quality
heavy
1/2 weight of glass
thin & light
high shatter risk
low shatter risk
very low shatter risk

Breaking a Cr-39 Lens

The Shatter Risk Myth

If you are low myopia and don't use rimless glasses, you might still be sold on the "upgrade" to a lower optical quality and more expensive lens, with the shatter risk sales pitch.

Is it really worth the higher price and much lower optical quality, though?

CR-39 has particular issues with cracking, if you drill them. Beyond that though, breaking this type of material requires an impact far higher than you are likely to ever experience. Even then, it does not shatter the way glass does. In my 40 years in the optometry field, I have never seen or heard of an eye injury from a shattered CR-39 lens.

Take a look at the YouTube video on the right. Look at just how much force he has to exert, on the whole of the lens, before it breaks (clean break also, no shatter). What would cause that much force? Nothing, most likely. Shatter risk is insignificant, by all practical accounts.

Aside from lenses themselves, you also want to educate yourself about myopia causes, and ways to prevent your eyesight from further deteriorating:

Google Scholarly Tells The Myopia Story

Source

Trivex Lens, And Alternatives

There is also Trivex now, quickly becoming an industry darling.

It combines the positives of polycarbonate with higher optical quality, which makes it a step in the right direction for lens prescriptions.

The main question should always be, what the biggest priority of your lenses is.

#1 Priority Should Be Optical Quality.

You can buy a great poly lens, but it will cost 10x to 40x as much as the equivalent in an inexpensive CR-39. If you are highly myopic, looking at the various high index options makes sense, to give you a thin and light lens, while reducing the optical quality compromise.

Lenses Tend To Cause Progressive Myopia

All prescription lenses, whether it is CR3-9, poly, or others, tend to cause progressive (lens-induced) myopia. Lens-induced myopia is the primary cause of human myopia above -1.5 diopters, and initial myopia is almost exclusively NITM (near induced transient myopia).

Your risk of retinal detachment increases 4x for mild myopia and up to 10x if you wear lenses over -5 diopters. Head over to EndMyopia.org to learn more about how to keep your eyes healthy.

15 comments

someonewhoknows profile image

someonewhoknows 2 years ago from south and west of canada,north of ohio

Poly ? want a "CRACKER"? Pun intended!


Sheila C 2 years ago

This is a fantastic article. I just had an optical score try to scam me into buying polycarbs for my -1.50 TV-watching glasses (nothing dangerous flying around in my living room!). When I said I didn't need them she said I would have to sign a waiver if I wouldn't buy them. I told her that was an UNETHICAL trick to try to scare me into buying something overpriced that I don't need. I will never shop there again and have already reviewed/complained about that store all over the Internet.


Tom 18 months ago

I have -3.50 in both eyes and have always worn glasses with polycarbonate lens. I haven't had any problems with polycarbonate personally. I decided however to try CR-39 on a new pair of glasses just to see if there was any real difference in optical quality since CR-39 has a much higher ABBE value that poly. I have read that some opticians consider poly "crap" and that some feel the only difference you will see with poly versus CR-39 is with high prescriptions which will introduce chromatic aberrations in the peripheral, something I haven't experience. Anyway, to my eyes, the vision CR-39 provides is way, way better than polycarbonate. My vision is far sharper with CR-39 in any direction including straight ahead. It's amazing. Now when I try my poly glasses it's like looking through fog. I'm changing all my glasses to CR-39. It is slightly thicker, but only slightly. Wish I would have known about this sooner.


FrauenfeldClinic profile image

FrauenfeldClinic 18 months ago Author

They're not malicious, it's what they're taught. However, most everything that happens during an optometrist visit, is SALES. Sure they call it "prescriptions" - but that's just to hide the fact that it's a commodity, they want you to buy it, that's how they make their money.

It's a slippery slope, the combination of "medical advice" and selling some of these products with the goal of making as much money as possible.


Tom 18 months ago

One thing I don't understand is this article here:

http://www.2020mag.com/l-and-t/12334/

Which is basically stating that an aspheric polycarbonate is as good or superior to CR-39 with visual acuity. And that polycarbonates flaws are only visible in areas that most don't see.

In my case, I've never had problems with polycarbonate with chromatic abberation (-3.50). But, I have four glasses all with the same script, one aspheric polycarbonate, one polycarbonate and one CR-39. The CR-39 is much sharper and clearer than the other two. It's obvious to my eyes that CR-39 is a huge improvement looking out of them in any direction include center.

The worst one is the aspheric polycarbonate. The side view is terrible. And supposedly this is supposed to have superior optics. The regular polycarbonate is better than this, but still not even close to the CR-39.


Tom 18 months ago

That should have read two polycarbonate to equal 4 glasses.


davidco 13 months ago

Hi, I also have -3.5 but no optician wants to put cr-39 in there as my glasses are without bottom rim and they say it will bee too thick and they also can not make the groove for string in that material. Can you please measure how thick your lens is on the side and bottom, as optician also can not provide me with that info. Thanks


Tom 13 months ago

I think there is an issue with CR-39 lens and rimless glasses. I believe they can crack easy in the process. So rimless and cr-39 is out.

I have a few different pairs of plastic rim glasses and cr-39 fits fine in all of them. The lens are slightly thicker and heavier than poly, but not by that much. After wearing cr-39 for a while now, poly carbonate looks really bad. It's not just the sharpness of vision, but the poly seems like it is uneven in strength in different parts of the lens.

It's great that CR-39 is much cheaper than poly, but i would pay more than poly for them.


Leon 13 months ago

Listen, i am an buyer in the optical industry and as follows:

Buying price of CR39 including ALL coatings is $0.83 a piece. Thats for all powers between +6 and -6 cilinder max 2. In Europa nobody uses poly. Nobody. For rimless they choose MR7 which is 1.6 index. Costprice for a MR7 lens is $1.65 a piece. MR8 which is used for 1.67 buyingprice is $3.0 a peace. Also including all coatings. They call that SHMC (superhydrofobic, hardcoat, multicoat which is AR and UV-400). Progressive lenses CR39 are about $7.- a piece for induvidual. Poly i don't buy but for 100% sure its far less than $2.0 a peace for single Vision. Consumers are paying far far to much for lenses and frames as well. Its just plastic! Yes we need expensive machines for the coatings (Satisloh) but without the coatings a CR39 is $0.40. And i am talking Essilor Factory products here which is super quality (60.000 employees and a market cap of about $30 billion on the CAC40 in France).


davidco 13 months ago

Those are insane margins! I see that MR8 has Abbe 41 which is still much better than 28-30 for poly.

It is strange that nobody wanted to make me cr39 at -3.5 as I used to wear rimless CR39 at -2.5 diopter for 3 years without ever cracking or chipping issues.

At the end I took Poly as I am in US and I needed the lenses urgently, but this is definitely worth switching on the first travel to Europe as the chromatic abberations and other optical degradation with poly even a few degrees off-center are very annoying.


Kay 10 months ago

Thanks a ton for this post - learnt a lot. Thanks again to Leon, Tom and others who shared valuable insight.


Patti Terry 9 months ago

Patti Terry...loved Leon's honest response about the cost of lenses! I've had a problem with going to different optical stores. Sometimes my lenses are not so magnified looking but others have. They choose what is best for them when u actually don't know anything about lenses. U put your trust in the experts. Bad thing...as I've found out for about 10 yrs. My last pair 3.50.. and had varilux physio2.0 with crizal cost me $855.00.

Thinking MR8 1.67 is a good chose but not available in Canada

Thanks again Leon for your insight..Patti


Ali 7 weeks ago

"All prescription lenses, whether it is CR3-9, poly, or others, tend to cause progressive (lens-induced) myopia. Lens-induced myopia is the primary cause of human myopia above -1.5 diopters, and initial myopia is almost exclusively NITM (near induced transient myopia)."

Looks like I have been the victim of the above said phenomenon, some years back I got a small myopia of about -1.5 which is constantly increasing every year without a reason. It is absolutely annoying to know that the very thing you use to cure myopia the lense is itself increasing it.


Bo Jangles. 7 weeks ago

Nice article, few people have ever heard of chromatic aberration, basically polycarbonate isn't blurring the way poor optics would blur, instead its blurring things the way a prism does refracting blue and red light differently, which your eye will still detect. This is the poor optics the good doctor is talking about.


Shrey 2 days ago

Thank you man for sharing :)

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