Eye Care

My prescription is -4 and -3.50 diopters with a cylindrical of -.50 on each. I plan on buying a plastic frame from zenni. Should I even bother upgrading to a 1.59 lens or is 1.57 and 1.59 barely noticeable in thickness?

I'm not an eye doctor, but I am pretty good at math. I'd say the difference is 0.02.

It's an interesting question and I'm guessing if I did a bit more searching around I could find more on the mathematical formulas behind lens thickness and lens material. The important factors I would think would be the lens thickness as well as the weight of the material. One interesting aspect of your question is that polycarbonate lenses are typically 1.59 index, yet Zenni has some sort of material they typically use with an index of 1.57.

Looking around, I did find this lens thickness calculator that you can try out. Your prescription seems to be in the medium range - mine is MUCH weaker, but many people have much stronger prescriptions. A stronger prescription will need a thicker lens (I'm not an eye doctor, but I'm pretty sure this is correct). The material of the lens determines how thin glasses can be in the center. A stronger material like polycarbonate can be cut thinner in the middle. The strength of the prescription combined with the refractive index (higher in high index lenses) determines how much thicker the lens needs to be at the edges. A larger lens will get even thicker at the edges.

The above referenced calculator will give you an estimate of the lens thickness at the edge. I just guessed at how to enter in your prescription and guessed at a pupillary distance and a frame eye size. I then selected the mid-range 1.56 index (which is close to the 1.57 of Zenni's lenses) and compared the numbers to the polycarbonate lens, which should be a 1.59 index. While I don't really know what I'm doing - it looked like the difference was about 0.2mm - which to me seems like it wouldn't be worthwhile.

Perhaps there is a real eye doctor out there that can weigh in on this one? I saw you posted a message on optiboards, but unfortunately, they don't allow non-professionals to post messages, as I'm sure you'd get an answer that you can be much more confident with from one of their users.

There is not much difference between a 1.57 or 1.59 lens. Usually with my patients, if they're prescription is -3.50 or higher I put them in a 1.67 hi-index lens to help with the thickness. But that's just what I do. Others may feel differently. There is also the difference between the materials. 1.67 hi-index is the plastic lens which if you're used to wearing a polycarbonate lens you may notice a difference in the clarity of the lenses. I know that I've switched back and forth myself and I like my poly lenses better. But I'm just used to poly more than the 1.67. I also end up picking out plastic frames for myself so the outer edge thickness is hid pretty well with that style of frame. Hopefully this helped and I didn't ramble too much.

george wrote:I'm not an eye doctor, but I am pretty good at math. I'd say the difference is 0.02.

It's an interesting question and I'm guessing if I did a bit more searching around I could find more on the mathematical formulas behind lens thickness and lens material. The important factors I would think would be the lens thickness as well as the weight of the material. One interesting aspect of your question is that polycarbonate lenses are typically 1.59 index, yet Zenni has some sort of material they typically use with an index of 1.57.

Looking around, I did find this lens thickness calculator that you can try out. Your prescription seems to be in the medium range - mine is MUCH weaker, but many people have much stronger prescriptions. A stronger prescription will need a thicker lens (I'm not an eye doctor, but I'm pretty sure this is correct). The material of the lens determines how thin glasses can be in the center. A stronger material like polycarbonate can be cut thinner in the middle. The strength of the prescription combined with the refractive index (higher in high index lenses) determines how much thicker the lens needs to be at the edges. A larger lens will get even thicker at the edges.

The above referenced calculator will give you an estimate of the lens thickness at the edge. I just guessed at how to enter in your prescription and guessed at a pupillary distance and a frame eye size. I then selected the mid-range 1.56 index (which is close to the 1.57 of Zenni's lenses) and compared the numbers to the polycarbonate lens, which should be a 1.59 index. While I don't really know what I'm doing - it looked like the difference was about 0.2mm - which to me seems like it wouldn't be worthwhile.

Perhaps there is a real eye doctor out there that can weigh in on this one? I saw you posted a message on optiboards, but unfortunately, they don't allow non-professionals to post messages, as I'm sure you'd get an answer that you can be much more confident with from one of their users.

I tried out the calculator and it also gave me a .02 difference on the edge, which seems like nothing. I guess I'll with 1.57 for now. Thanks for the url and your reponse.

alpjeffrey26 wrote:There is not much difference between a 1.57 or 1.59 lens. Usually with my patients, if they're prescription is -3.50 or higher I put them in a 1.67 hi-index lens to help with the thickness. But that's just what I do. Others may feel differently. There is also the difference between the materials. 1.67 hi-index is the plastic lens which if you're used to wearing a polycarbonate lens you may notice a difference in the clarity of the lenses. I know that I've switched back and forth myself and I like my poly lenses better. But I'm just used to poly more than the 1.67. I also end up picking out plastic frames for myself so the outer edge thickness is hid pretty well with that style of frame. Hopefully this helped and I didn't ramble too much.

It did, thank you. If you don't mind I have a couple of quick questions I'd like your opinion on. I went in for an eye exam not that long ago and I noticed that when the optometrist was telling me to read the eye chart I couldn't focus on the last and smallest line of numbers to tell what they were. I assumed the optometrist would keep switching to a stronger lens so I could focus in but after one or two tries they stopped. Is there a reason for this? Isn't the object for me to get as close to 20/20 as possible?

Blindman wrote:Is there a reason for this? Isn't the object for me to get as close to 20/20 as possible?

some doctors will go as far as using the 20/15 line during their exams, which is smaller than 20/20. So if he/she can get you to read a few letters that small even though you can read all of them it's good that you can aleast read a couple. There is really no way to tell if this was the situation unless you ask the doctor who refracted you.

Seems to me the real difference isn't captured by those numbers at all. The refractive index 1.59 is "pure" polycarbonate material, while the 1.57 is a "composite" of polycarbonate and typical plastic. One is slightly thinner and lighter than the other, but as noted the difference is very small, too small to guide a decision. (On the other hand, seemingly small differences in the index of refraction number make a really big difference, and the calculated percent difference numbers aren't representative of what you'll actually see. Non-glass lense index of refractions vary from 1.498 [very "thick" glasses] to 1.74 [very "thin" glasses], more of a difference than you'd expect from the numbers alone.) Polycarbonate is especially used for safety glasses, as it doesn't break easily even when dropped, and as it blocks ultraviolet even without any coating. Its downsides though are it scratches more easily and it's more subject to "chromatic aberration" (colored "fringes" at the edges). The "composite" probably has a little less of both the pros and the cons of pure polycarbonate: breaks a little easier (but still not very easy), doesn't block UV quite so thoroughly (but that doesn't matter with a coating), scratches a little less easily (yay!) and is a little less subject to "chromatic aberration" (again yay!). There's also most likely some difference in price.