Help me figure out how thick my eyeglass lenses will be.

June 26, 2015 11:00 AM

I'm trying to decide between a bunch of lens options, materials and index. Of course, I'd like to get the thinnest lens I can without spending more than I have to and crossing the line of diminishing returns. Surely it's possible to know exactly how thick a lens of a given material/index with a given prescription will be. Help me find out how to determine this.

It seems that all the information I can find about "what lens is right for your prescription" is handwaving along the lines of:

Zenni: for prescriptions with a spherical power above +2.00/-4.00, a cylindrical power above +/-4.00, and an ADD power of +2.50 or higher, selecting a 1.61 or 1.67 index lens can result in thinner lenses in your glasses.

OR

highindexlenses.com: Lenses with an index of refraction of 1.70 or higher typically are at least 50 percent thinner than conventional plastic lenses."

For example, Zenni offers:
$27.95 1.50 Digital Free Form Progressive (No-line multi-focal)
$27.95 1.57 Digital Free Form Progressive (No-line multi-focal)
$55.00 1.53 Trivex Digital Free Form Progressive (No-line multi-focal)
$45.00 1.59 Polycarbonate Digital Free Form Progressive (no-line multi-focal)
$55.00 1.61 Digital Free Form Progressive (No-line multi-focal)
$73.00 1.67 Digital Free Form Progressive (No-line multi-focal)

And for my prescription they recommend the first. They claim their automated lens-picker chooses the best one for thickness. I'm currently wearing a pair of Zenni 1.67, which don't seem incredibly thin, so it seems like those 1.50's would be much thicker. Can't find out without ordering them though.

Today, someone at highindexlenses.com told me:

You would be good with the High Index 1.74

and gave me some interesting information on the minimum thickness of lenses at various indexes and materials, but didn't have any specifics beyond that.

Now, all these lenses get cut from round blanks, and it's all just math, so it seems to a layman like me that for a given material/index and prescription that the lens profile can be calculated in advance.

Take a look at this chart:

In the case of the first line, there's obvious value from 1.50 to 1.57, but diminishing value above that. In the case of the third line, there's less apparent value as you go higher.

I'm looking for a resource that can tell a guy "This prescription cut from this material will be XX mm thick at the center and YY mm thick on the edges [before being cut down for the frame, of course]". Is there such a thing?

I mean, I can order six different pairs in six different indices and compare them myself, but surely there's a way to know, not just guess.

Maybe some lens-design software of some kind? Mathematica?

My prescription, in case it has some value in answering:
OD - Right +4.00 -2.50 110
OS - Left +4.25 -3.00 69
bifocal +2.50

posted by chazlarson to Clothing, Beauty, & Fashion (12 answers total)

Response by poster: A tidbit, from the PDF that the highindexlenses folks sent me:

• 1.74 high index plastic is slightly thinner than 1.70. This should only be chosen with very high prescriptions (stronger than +/- 10.00) or when the lens material you want is not available in another index.

I don't fall into that prescription bucket. That, of course, makes me think they're recommending it based on their margin, not the properties of the lens. The booklet is titled "HIGH INDEX LENSES & GLASSES: The diamond ring of eyewear." so perhaps I can look forward to it suggesting I spend two months's salary on my lenses.
posted by chazlarson at 11:13 AM on June 26, 2015

The size of the lenses is also a huge consideration. At the same prescription and type of material a larger lens will be thicker than a smaller lens.

Anecdata: my prescription is worse than yours and there's a noticeable difference between 1.67 and anything less than that, but the difference between 1.67 and 1.74 is subtle.
posted by kbuxton at 11:17 AM on June 26, 2015

Response by poster: I guess I was under the impression that lenses all got cut as circular blanks of the same size and then got trimmed to fit the frame. That's certainly the impression that the Zenni "Production story" slideshow gives. The lenses are all produced as circular lenses with a diameter of what looks like about 4-5 inches, then they get trimmed and edged to fit the frame.

Sure, once it's cut down to fit the frame, the edge thickness will vary, but if two people order the same prescription in the same lens material, at the end of the lens production step those two pairs of lenses are identical. That's my impression. Perhaps that's mistaken.
posted by chazlarson at 11:26 AM on June 26, 2015

Mod note: This is a comment from an anonymous answerer.

Now, all these lenses get cut from round blanks, and it's all just math, so it seems to a layman like me that for a given material/index and prescription that the lens profile can be calculated in advance.

Maybe some lens-design software of some kind?

Uh, speaking with specific knowledge on this topic (I write that kind of software that calculates this stuff), yes it is "just" math. But it is a lot of math that also involves the size and shape of the frame, the size of the add-power segment, the style of the lens, manufacturer of the style of the lens, etc. Additionally, the options you list are all digitally surfaced (versus standard toric) which means they require more precise measurements and possibly additional inputs for your precise prescription. There are literally TENS OF THOUSANDS of SKUs for lens blanks. Different lens surfacing labs will choose different lens blanks. Different digital calculators will choose different lens blanks.

Go to an optical store and talk to an optician there. They can give you much more accurate answers, especially with regards to the frames you are looking at, than anyone here. That is their job and they are trained to do it.

posted by cortex (staff) at 11:31 AM on June 26, 2015 [4 favorites]

The outer edges of a circular blank are thicker than the middle, so the thickness at the edges of the lens will differ depending on how far out they are from the center.
posted by kbuxton at 11:32 AM on June 26, 2015

In this prescription the overall distance power is plus, so the outer edges of the circular blank are actually going to be thinner than the center. Frame selection will be a factor since the center of the lens may have to be thicker to leave enough material on the outside of the lens to create a groove. Without selecting a frame and speaking with an optician there is no way to tell exactly how thick your lenses will be. As for deciding lens options, the design of the lens is probably going to play a much larger role in your ability to use the glasses than any other choice. An optician should be able to walk you through your lens options and help you choose a frame that will work well with your prescription.
posted by JFI at 12:09 PM on June 26, 2015

Response by poster: I'll note that I do not mean to disrepect anyone. I'm speaking purely as a guy who has worn glasses for 50 of my 53 years, and has never, at any of the dozen places [ranging from a local optician to costco/walmart to lenscrafters or tuckerman to online sources] where I've bought glasses, been able to get any more specific information than "these higher-index more expensive lenses ought to be thinner". At at least a few places, I've specifically asked "Is there a way to know for sure?" and invariably been answered with "no".

So I pick one, then get it, and they're as thick or thin as they are, and there's no way to know exactly what that extra $100 or whatever bought with regard to the lens thickness or weight.

It just seems to me that this ought to be more of a "will be" or "won't be" question rather than "might be" or "should be". Apparently it doesn't work that way, which surprises me. Or maybe I've just been unlucky in the places I've chosen to go.

Not trying to be sarcastic. Sincerely looking for information on what has, for 30 years or so, been the single most stressful aspect of buying glasses in-person or online. Sorry if I touched a nerve somehow.
posted by chazlarson at 12:41 PM on June 26, 2015

The 1.50, 1.57, 1.74, 1.67, etc, are all the indexes of refraction for the lens material. The different materials are also easier/harder to tint or coat, or do/don't require additional coatings due to how soft or hard the material is, how heavy/light the material is, require different processes to grind, have more or less impact resistance, etc. There's a reason why there are so many materials and why there is such an industry surrounding it - there just aren't easy answers.

But, here's the first google result (other than ads) for "optical lens thickness": Lens Thickness Calculation. Looks like you can plug your prescription and frame size in there along with choose different lens materials to compare. "Actual thickness may vary depending upon the specific lens and frame style."
posted by jillithd at 1:24 PM on June 26, 2015

If your prescription isn't too unusual, you could possibly go to a shop that makes their glasses (i.e. cuts lenses and puts them in the frame) on-site, and ask the optician to show you the blanks. Mr. Tech used to be an optician, and he'd have totally be into that kind of geeking out over lenses. (If your prescription is weird, they'd have to mail-order them in, and would probably not be willing to do that just to satisfy your curiosity.)
posted by BrashTech at 1:25 PM on June 26, 2015

If we were to remove the variable of the add power and look at it as a single vision lens, it would be possible to closely estimate the center thickness of the lens. The information needed would be your prescription (which you provided), your PD (pupil distance), the base curve of the lens and the frame measurements. To give you an example, if you know that your PD is 64mm, and the frame has a 18mm bridge and an eye size of 55 and is a traditional shape (not too squared or round), and your prescription is +4.25 - 3.00 x 69 (i'll just use one lens for the example here), and the bc of the lens is a 6, then we could determine that a lab with high quality control should produce a lens with a center thickness of approximately 5.5mm in regular plastic. Upgrading to a high index 1.60 would make it closer to 4.8mm. 1.67 would be 4.3 and 1.74 would be 4.0 (roughly).

Frame selection has a much higher chance of reducing your center thickness. For example, the same rx in plastic but with a frame eye 52mm and a 16mm bridge could be made with closer to a 4.5 mm center, as opposed to the 5.5 mm in the larger frame.

Of course these are ideal situations and still not exact. So it would be possible to get a relatively close estimate given enough information. But once you throw the add power +2.00 in the equation then all of this math gets thrown off.
posted by JFI at 1:36 PM on June 26, 2015 [1 favorite]

Response by poster: Thanks, everyone; I've learned a lot here.

jillithd, I think that's exactly what I had in mind. That sort of relative comparison between different materials for a given prescription and lens size is what I have been missing. Holding everything else equal, going from a polycarbonate lens to a 1.67 index lens goes from Center 3.9, Edge 2.6 to Center 3.6 Edge 2.4. The "Hard Resin", which I imagine is the basic "Plastic lens" is about 1mm thicker both places. That's something that can help make an informed decision.

JFI, thanks for the walkthrough.
posted by chazlarson at 1:53 PM on June 26, 2015

Response by poster: Closing the loop:
I now have two pairs same prescription, one 1.57, one 1.67:

Slightly different frame. I don't have a caliper handy, but based on the way they feel side-by-side between my fingers while cleaning them, the 1.67 are a bit thinner overall and maybe have a bit more of a "scoop" to the shape. Wearing them, the 1.67 distorts a little more. Hard to describe, but the 1.57 feel like "I can see clearly" and the 1.67 feel like "I can see clearly because I'm wearing glasses".
posted by chazlarson at 9:45 AM on July 16, 2015