Should You Buy CR-39 Lenses Instead of Polycarbonate?

Ling writes:

quotesAlex, I am shopping for a new prescription and am confused from the options.  I have reduced to -1.50 and can see about 20/25 with them.  The optic shop says to buy polycarbonate high index, but since my eyesight improves a lot with your program, I wonder if they are worth the extra money.  What do you think?

This is a fairly common question, so let’s look at it here briefly.

The type of lens material you want to use, depends on a variety of things, including:

  1. Prescription Strength
  2. Intended Use / Exposure
  3. Frame Type
  4. Budget

Ling has a very low prescription strength, which is a major deciding factor in the type of lens material to use.  She is also not going to engage in activities where sharp objects are likely to fly rapidly at her lenses.  The budget is important to her, though we don’t know about the type of frame she wants to use.

Let’s look at each component of the lens choices:

Prescription Strength

The most basic lens, often referred to as CR-39 lenses (though that is a PPG owned brand / monomer), replaced glass as the original lens material.  

Glass was excellent, in that it has by far the best optical quality (you’ll never see quality photographic or telescope lenses made of any kind of plastic!).  It is also not easily scratched.  Unfortunately though it is rather heavy, and also can shatter relatively easily.  And most of all, it gets thick quickly, as you increase the diopter values.

Because of weight and shatter risk, glass is rarely used in prescription lenses these days.  

That is where CR-39 lenses (Wikipedia link) improve on glass.  It’s optical quality is almost as good as glass (and far better than all the other options we will look at), it is half as heavy as glass, it is much more resistant to shattering, and still very resistant to scratches.

cr-39

In ‘raw’ form, before cut to your frames – CR-39 lenses.

It is also inexpensive to manufacture (and due to the nature of the manufacturing – pouring the monomer into optical glass molds it takes on the properties of the glass).  Further, it also takes well to being dyed (for sunglasses) – much more so than polycarbonate.

So all in all, the inexpensive CR-39 lens is a great option.  Then, why does the optic shop want to sell her high index poly lenses instead?

High index translates to a thinner lens at higher diopters.  So to avoid having coke bottle glasses, you can go with higher index, when you need a high diopter prescription.  More on the subject of high index, in this article.

But at low prescriptions, especially at -1.50, the lens is thin anyway.  High index will make no difference.

If your prescription is below -2 diopters, we don’t need high index lenses.

And the optical quality of a good CR-39 lens, even though it’s cheaper than all the alternatives, tends  to be superior, sometimes noticeably.  I almost always go for the CR-39, when dealing with low prescriptions for that reason alone.

There are two notable exceptions:

Intended Use

Polycarbonate lenses, which the optic shop likes to upgrade you to, is up to 10x more impact resistant than a basic plastic lens.  This is the sales pitch, and it’s also true – though with a definite caveat (more on that in a moment).

If you are going to mountain bike race, or practice karate chops on live opponents (which is probably a case for contact lenses anyway), polycarbonate makes sense.

But 10x more impact resistant is a bit misleading.

A CR-39 lens is quite impact resistant.  If you live a regular lifestyle, or wear your glasses just for the regular lifestyle aspects, being concerned about a shattering lens might be overblown.  In all my years I never even heard of a case where a plastic lens caused any injuries.  And remember, it’s far more impact resistant than glass – for 99% of applications, it’s a perfectly safe choice.

But if you have to duck flying debris on a regular basis, think of polycarbonate.

On the downside, the polycarbonate has (sometimes notably) inferior optical quality.  Compared to a glass lens, the performance of even the high priced poly options are less than stellar.  And while it’s debatable how much this matters, I am a stickler for optical quality.  Remember, the primary cause of myopia above a -1.5 NITM case (the precursor to real myopia), is always lens-induced myopia.  It’s the glasses that shape the change in our eyes.  And when we do rehabilitative therapy, we focus on simplifying prescriptions and improving focus.

cr-39-vs-poly

Thinner, lighter, more impact resistant.  But also, an optical quality compromise.

A poor quality lens is not helping our cause.  Do you really need 10x more shatterproof-ness, in exchange for a degraded image reaching your brain?  

It’s not a compromise I like to take.  If you have a higher prescription and need to go with high index, this is a bit more of a complicated subject.  At that point we could talk about the benefits of various lens technologies, coatings, and how different manufacturer approach the challenge of good optical quality.  For Ling’s case though, definitely, so far CR-39 lenses are our first choice.

Though I did say, there are two exceptions:

Frame Type

The problem with CR-39 is, that it doesn’t lend itself so well to changes to its structural integrity.

In other words, drilling holes into this type of lens, as you would need for a frameless design, tends not to work out so well.  CR-39 lenses like to crack along drill holes, so optic shops tend not to accept these lenses unless you get a full frame.

frameless

Not a setup for CR-39 lenses.

That’s the question again – is there a frame you will like, that won’t leave you compromising for optical quality?  Even I contradict myself here, as you may have read in my favorite frames article.

If you have higher myopia and need a high index lens, this might be a non-issue.  You will probably go with a lens type that drills well, and this won’t be an issue to sort out.  Then though of course there is a higher expense for lenses, and … it isn’t really Ling’s question, or:

Budget

There are other options, of course.  Trivex, with better optical quality than polycarbonate, but still high impact resistance, is becoming an industry darling.  Of course, it’s not necessarily so much budget friendly.  

Also, the Internet, if you do some digging, provides some great discounts on high quality lenses.  This is a subject I keep meaning to cover, since you can separate your frame purchase from your lens purchase, and avoid overpaying for either.  If you are in Western Europe especially, there are several optic shop chains that absolutely work with you, if you choose to BYOL (bring your own lens!).  This too, a topic for another article.

So, What’s the Verdict for Ling?

If Ling doesn’t need a frameless setup, the CR-39 lens will suit her perfectly.  Absolutely add anti-reflective coating.  

Plastic_CR39_Transitions_Signature_VII_Lenses

I personally would also consider Transitions brand new Series VII (humorously terrible video production here, though the product is excellent) photochromic coating – it is nothing short of brilliant.  

If you are not familiar with photochromic, it is a coating that automatically darkens your lenses when you go outside.  The brighter the sunlight, the darker the lens get.  Once you go back indoors, the lenses return to being completely transparent.

VII gets much darker even in high temperatures (up to 85% – quite amazing), it works better with indirect UV than the previous generations, and it clears faster when you go indoors.  It’s a strong improvement over the Series VI (which was already impressive), and in a CR-39 lens it can be as little as a 20-30 Euro upgrade to your lens purchase.  The increased contrast also improves the image, another bonus for those of you who are working on active focus, outdoors.  And unlike the early versions, today’s photochromic lenses get perfectly clear indoors.

I’m in no way associated with PPG or Transitions Optical – this is merely my opinion of a product that may warrant consideration.

I will keep the subject of lenses on the list of topics, so if you are interested in what goes in front of your eyes, keep the blog in your bookmarks!

Enjoy,

 alex cures myopia

 

2016-11-09T02:20:00+00:00 By |Categories: #endmyopia project News, Focal Planes|