1.67 Index R 1.50 Index L Question

[John@OWDC]

I got this question:

Hello John,

Hope all is well. Here in Suriname all is ok. Busy. I'm now busy making a workshop and i am in need of some help. I cannot find it on the internet. Hope you can help me.
Case:
RS -6.50 C-1.25 ax 160
LS- 1.50 C-0.75 ax 180
Salesrep advised 1.67 material for right eye and 1.5 for left eye. That is impossible, as you know. (the thought behind this was: R-lens and L-lens could be equally thick now.)
Ok, so far the case. I want to explain to the group (45+ members) what happens if you'ld really sell this. In my mind, that is not a problem. What I am looking for is: an image where you can see a ray of light going through two different materials. Do you, by any chance, have such an image? And: would you be so kind to send this to me?

We have been going back and forth with it and I said I would toss it up here and see if we are lucky enough to grab an old-timer who can (really) explain why you would or why you could or why you would not or could not or should not.

Thanks

[drk]

I see two potential reasons to do this.

1. save money on one lens. Sure, why not? I mean, if it's just factory-coated CR39 vs. 1.67 without coatings, it is do-able.

2. try to make the lenses equal thickness. Well, if you ran the numbers on how thin the OS would be 1.5 vs. 1.67, the difference would be minimal. So, in the (misplaced) idea that you'd be "keeping the OS from being too thin", you'd be wrong. It isn't an issue. If you want to make lenses EQUALLY THICK, well, then, just order 1.67 OU and add CT to OS.

But in reality, why would you do this? The lenses will yellow at a different rate. The edge luster will be different, I'd think, polished or not. The reflectance will be different-looking. I'd guess that if AR-coated, it would look a little different, too.

It's a dumb idea, overall.

[John@OWDC]

Keeping in mind please I am only the messenger here -

I'll add a little bit more and see if it takes us anywhere else.
Let's say the underlying questions are these:
"Is there any optical reason not to?"
OR
"Is there a physics of optics reason not to do this?"
OR
"Is there a refraction/refracted ray reason not to do this?"

[Ruud van Pel]

DRK,
I’ve asked John to help me. Let me explain some more: i am
organising a workshop overehere. And this is just a case, what really happened. I want to use this case, just to let my coworkers be aware of the things they are thinking of. What i actually looking for, is an image, where you can see the difference in refractive breaking between two different breakingindexes.

I see two potential reasons to do this.

1. save money on one lens. Sure, why not? I mean, if it's just factory-coated CR39 vs. 1.67 without coatings, it is do-able.

2. try to make the lenses equal thickness. Well, if you ran the numbers on how thin the OS would be 1.5 vs. 1.67, the difference would be minimal. So, in the (misplaced) idea that you'd be "keeping the OS from being too thin", you'd be wrong. It isn't an issue. If you want to make lenses EQUALLY THICK, well, then, just order 1.67 OU and add CT to OS.

But in reality, why would you do this? The lenses will yellow at a different rate. The edge luster will be different, I'd think, polished or not. The reflectance will be different-looking. I'd guess that if AR-coated, it would look a little different, too.

It's a dumb idea, overall.

[Robert Martellaro]

Mixing refractive indexes resulting in different Abbe values might cause discomfort due to color fringing and off-axis blur in one eye. Different refractive indexes may also force the software to use less than desirable base curves, and possibly different aspheric/atoric P-values, resulting in discomfort from magnification effects.

The above ignores the elephant in the room, that is, 5 D of aniso- the above may all be inconsequential depending on the cause. And yes, I've mixed materials with success but in most cases there was some degree of vision loss in one eye, or lack of binocularity.

Best regards,

Robert Martellaro

[drk]

I can't see any theoretical reason it would not work.

I don't understand what you are looking for with ray tracing. Any lens with the same power is going to have the same ray trace (in an idealized thin-lens, paraxial way that we do it). Regardless of index, thickness, regardless of front and back curve. It's just the back vertex power that counts.

Now if you want to get all precise, then I'm sure there's more off-axis aberration in a higher index lens than a lower index lens (despite lens design compensations), but the germane comparison would be -6.00 BVP with Cr39 versus -6.00 BVP with 1.67.

Not -6.00 BVP 1.67 vs. -1.00 1.5. Why bother to compare off-axis aberration between those? They'd never be the same under any condition.

I think you're wondering if there is MORE of an off-axis aberration DIFFERENCE with DIFFERENT materials, and LESS of an off-axis aberration DIFFERENCE with SAME materials. It's probably true to a very small degree, but it's less of a concern than the practical concerns I mentioned in the first post.

And to Robert M.'s point, in the real world, there are so many other problems with wearing a 5D difference in lens power that this is probably minutiae.

The reason to do it, or not to do it, is more a practical matter than a theoretical one, in my opinion.

[drk]

Per Darryl's Opticampus, given average lens decentration parameters (60 p.d., 52-18 frame, and a 1.5 center thickness to be consistent with 1.67) a -1.50's ET with CR-39 is 3.1mm.

Running the calculation with 1.67 gives 2.6.

The difference is an invisible 1/2 mm. This means the idea of leaving the lower power lens in a low index lens to match the thicker lens is an invalid suggestion in the first place.

In contrast, the -6.50 lens' ET in 1.67 is 6.4.

One eye: 6.4
Other eye: 3.1 or 2.6.
It's the same thing, really.

[Lensman11]

I must be missing something on this one. I will say never a day went by that someone did not ask to match edge thickness because of Rx disparity. Sometimes with two indexes as this case or increasing the thickness on one to match the edge. This never poses an optical problem because it is invariably a patient request so they are happy.
calculated this one with 52 eye no decentration right will be 6.5 edge is 3.7. You cannot produce cr 39 under 2.0 center it will pass drop ball testing but you can increase the center to 3.5 making the edges closer. I might get some push back on this but if it had decentration you decenter the weaker more than required and the stronger less but not beyond ansi standard of 1 mm in each eye.

[ChrisBowers]

Everything said is true, we could move this to other solutions. Slab Off, Lasik or contacts. I just do not see spectacles as a satisfactory solution.

[juno]

Basically it boils down to: Can you do it? Yes. Will it cause issues? Probably not any more than the Rx disparity would anyway

[drk]

Lensman, since you have done this many times, obviously it's OK. But technically it's not OK to add CT to the lesser power lens, because you are creating (more) anisekonia. With anisomyopia you want equal BC and CT to keep the shape magnification difference low.

Now, having said that, it's just the geeky theoretical reason to not do it.

I've NEVER added CT to a weaker minus lens to equalize edge thickness. I HAVE added CT to an anisoplus prescription, though.

[Lensman11]

This was always done at patients request to make them feel better about the appearance of their glasses. What would you if the patient asked can’t you make my glasses the same on the edge.. no it’s because of your Rx and they say I have had it done before.
today their are many more options at your disposal in designs and materials than in years gone bye. It was fairly common to prescribe Ultex
bifocals for plus Rx because the oc was at the bottom of the seg and it would reduce image jump at near. Everyone knew this and did it routinely until the exec came out with oc on the seg line because everyone loved the wide reading field. Whatever the patient thinks is best for them they will be happy with it.

[Barry Santini]

An optician always thinks optics first
And cosmetics second.

B

[optical24/7]

Lensman, since you have done this many times, obviously it's OK. But technically it's not OK to add CT to the lesser power lens, because you are creating (more) anisekonia. With anisomyopia you want equal BC and CT to keep the shape magnification difference low.
.

You would treat anisekonia ( with eye glass lenses) the same as you would treat anisomyopia since they are the same, difference in image size, with Iseikonic lenses. Different power lenses have a degree of magnification or minimization. To increase a lens magnification we can increase center thickness, use higher than best base curves and extend the vertex difference. To decrease a lens magnification we can reduce center thickness, use flatter than best base curves and shorten the vertex difference.

Unfortunately, Iseikonic lens go counter intuitive to what we want; thinner higher powered lenses. In the above power example, we would need to make the higher powered lens thicker, higher base curve, and longer vertex, To decrease the higher power lenses’s minification. To get closer to the minification of the higher power, we would make the lower powered lens minification more; thinner, flatter base curve and shorter vertex. You can address this further by mixing indexed materials ( not by a lot). Not pretty, but it does address anisekonia.

As far as the original OP, as Robert pointed out, optically you can use mismatched indexes, but you could run into chromatic aberrations eye to eye.

[Lensman11]

One option not mentioned and readily available. Make the right aspheric and thin the edge for cosmetics not optics. Patient will like the cosmetics and just have to accept a reduced field of view.

[drk]

24, right.

Iseikonic lenses are one thing (bad) but minimizing aniseikonia with normal lenses (good) is preferable and actually practical. The bottom line: "equal base curves and center thicknesses".

[optical24/7]

24, right.

Iseikonic lenses are one thing (bad) but minimizing aniseikonia with normal lenses (good) is preferable and actually practical. The bottom line: "equal base curves and center thicknesses".

Doc, could you expand on this? Particularly minimizing aniso by equal BC’s and CT’s with a minus Rx as above.

[Kwill212]

Doc, could you expand on this? Particularly minimizing aniso by equal BC’s and CT’s with a minus Rx as above.

Sign me up for this TEDx Drk talk as well.

[Robert Martellaro]

He's thinking plus lenses.

Tips: Use a minus (concave) base curve and increase the Center Thickness to increase minification (or decrease magnification), the opposite of what one would expect when increasing center thickness. If you do not want the CT to have any effect on magnification, use a plano base curve.

Robert Martellaro

[optical24/7]

He's thinking plus lenses.

Tips: Use a minus (concave) base curve and increase the Center Thickness to increase minification (or decrease magnification), the opposite of what one would expect when increasing center thickness. If you do not want the CT to have any effect on magnification, use a plano base curve.

Robert Martellaro

Yep, plus vs minus have differing abilities in inducing magnification or minification. As example, you can effect way more magnification than minification with ct alone.

[drk]

[drk]

Ok, so the power factor can't be played with, without changing the focus of the lens. (It's OK we we do it, though!) You only get to play with the shape factor.

Now, in an isekonic design, you'd be messing with the thickness, index, base curve of the good lens to overcompensate for the bad lens, (that is, make a less plus/more minus lens MORE magnifying than normal to match up better with the big bulbous buddy), and even trying to slim down the bad eye to meet up with the good eye's magnification.

But if you don't want to get all fancy with iseikonic design and make a mess in the lab, then consider merely working with F1 (base curve) and t (center thickness) of the better lens.

WORKING WITH THE BETTER LENS:
If you get all "math-y", notice that if you could get the denominator in the shape factor to be 0.00000000001, then you'd get some serious magnification!

Mathematically if we can get the t/n x F1 term to go to "1" (high school algebra/baby calculus concepts...) the denominator approaches zero, and the shape factor is 1/0.0000000001, which means the shape factor goes to infinity magnified. That's a lot of magnification. Probably too much.

But do the realistic math. Say you have a +6.00D in the bad eye, and a +2.00D in the good eye. You want to reduce the magnification difference. You can't change the lens power because those doctors and such. So you opt to do the best you can without dealing with iseikonic design (yuck).

Say you're using spherically-surfaced 1.5 material, say the "corrected curve chart" says that a +2.00D should be on a +6.50 base, and that the frame size/decentration dictates that the center thickness would come out to 3.5 mm (totally made up). What kind of shape factor magnification would you get? I did the math: 1.015 X. ("X" as in, 4X magnification.)

Ok, what if you decided to spice things up around your optical? The bad +6.00D eye would have, say a +8.50 BC and a CT of 8mm. The shape magnification from GOOD EYE using those bad eye base curves and centers (nice and balanced-looking but thicker though but, suppose you care about aniseikonia in the first place or we wouldn't be talking about this and we'd be *****ing about essilor) is...1.05x.

So you went from 1.015X to a new 1.05X! That was fun. So, is that a big help?

What percentage of difference matters? About 2% difference in image size is a problem. (Robert Martellaro has been telling us size matters!) So, you've made a 10% improvement in the situation by doing that.


NO WAY, you say. I'm not doing that crap. That's why these guys make the big bucks (ignore the stupid term "dynamic aniseikonia" because it's stupid and they're conflating stuff) https://shawlens.com/aniseikonia/what_is_aniseikonia/

And these guys are researchers, so I know you'll trust them more than me---> https://www.sciencedirect.com/topics...ry/aniseikonia

[drk]

Yep, plus vs minus have differing abilities in inducing magnification or minification. As example, you can effect way more magnification than minification with ct alone.

I think what you're thinking of is that all minus lenses have equal center thicknesses, so that's solved.

Also, best-form base curves for minus lenses are much closer to the same for say a -2.00 and a -6.00, when compared to the best-form base curves for +2.00 and +6.00.

So, really, there's not a lot you need to do in anisomyopia, typically. Having said that, most of the crazy acquired aniso disasters are the anisomyopia kind...scleral buckles, nuclear cataracts (get surgery!), so you really have trouble with these kinds. Usually you undercorrect and life is good.

The anisohyperopes are born that way, and they are adapted to whatever, even if it's a sad adapation of weak fusion or central suppression or amblyopia. Those kids are the ones that really need iseikonic designs, but who wants their kid trashing $350 Shaw lenses? But it's the right thing to do, actually, and just passive correction alone can eliminate anisohyperopic amblyopia.

[Robert Martellaro]

Those were two of the best walk-around posts I've seen in some time.

You spelled my name incorrectly.

Robert

[drk]