Aspherics and power variation in periphery.

[kisselt]

Hello, I am an optician apprentice and lab technician in lenscrafters, (at least until they close all the labs soon) and I have a question about aspheric lenses. I am a hyperope, so I'm sure you all know I have a plus prescription. Irecently had a new pair of glasses made with aspheric lenses, and I am used to spheric lenses, however I notice a discernible lessening of power in the periphery. I am able to accommodate to counter it, but I was under the impression that aspheric lenses are supposed to actually have a more uniform power across the entire lens. I verified with the lensometer and the power does in fact gradually change, at about 10 mm from the optic center the power is about .25 diopter lower than in the center, and gradually reduces to about .50 -.75 less than the prescription at the far edges of the lens. I know that with spheric plusses the increase in vertex distance creates a slight increase in power in the periphery, and that the changing front surface of the lens of an aspheric is supposed to correct this. So shouldn't the power be uniform across the entire aspheric lens and not reduced by a quarter to half or even .75 diopter as I look in the periphery? Also, they were made from semi finished lens blanks with a 4 base, but isn't the change in the front surface of an aspheric lens supposed to be modified depending on the prescription? Like for a plus lens the surface becomes flatter in periphery and steeper in minuses? So doesn't using a semi finished lens defeat the purpose of an aspheric design? at least as far as power changing in the periphery is concerned. I mean if you use the same blank to generate a plus and a minus prescription, wouldn't this cause the change in power I'm seeing in the periphery? It seems like it's the exact opposite of the result I should be having with an aspheric lens.

[Hayde]

It's important to remember when we're talking about distortion in an aspheric lens, the basis of comparison is against a spherical lens in the same Wrong base curve. In that case, the aspheric (traditionally surfaced) will provide a superior field of vision with the correct power.

In terms of patient adaption, you'll be hard pressed to find a patient who finds the vision inferior to a conventionally surfaced spherical lens in the proper base curve, even if you detect a differential in the margins under a lensometer.

Welcome to Optiboard, Kisselt!

[Tallboy]

Exactly what Hayde said. All the aspherics available in frontside molds are using Asphericity not to improve vision on a best form spherical lens, but rather to make a way to flat lens wearable(bearable?) to the glasses wearer. To utilize Aspherics in a way that improves upon the peripheral vision in the correct base curve, to give better off axis vision than regular best form spherical grinding, there are no such custom front aspheric lenses. However there are lenses that utilize this asphericity on a best form lens when the asphericity is ground in the backside - these would be found among the varying digital lenses of the day. To see what asphericity alone does to improve vision choose a lens like the Shamir Spectrum SV or which is backside aspheric but does not alter the prescription such as an autograph, Zeiss, IOT or your Lenscrafters HD lenses. The benefit found in the Spectrum SV would be only from backside asphericity.

Or at least I think I've read that stuff on here before somewhere. Welcome to Optiboard.

[kisselt]

So essentially the distortion in power I'm seeing is because of an incorreCT base curve? The prescription is +1.75 -.25 156, on a 4 base, so a higher base curve would be a better choice? I'm still unclear as to why I'm experience a noticible difference in power outside the optic center compared with the spherical lens of the same prescription on the same base curve. Is that just to be expected because of the flattening of the front surface away from the center in the aspheric? And if so would a higher base curve be better?

[kisselt]

So an aspheric base curve on a semi finished lens that isn't digitally optimized on the back side is basically for a thinner lens? Not for better peripheral vision?

[Hayde]

So an aspheric base curve on a semi finished lens that isn't digitally optimized on the back side is basically for a thinner lens? Not for better peripheral vision?

Bingo! The digital lenses may advertise superiority on both counts, but I'd hesitate to sell them as such unless powers are rather strong and you're very proficient at VWT measurements (aka POW measurements.) Digital in and of itself doesn't make anything better--it's the 'secret sauce' computations those digital products are using that may (or may not) be worth the price bump.

[Tallboy]

So an aspheric base curve on a semi finished lens that isn't digitally optimized on the back side is basically for a thinner lens? Not for better peripheral vision?

Believe me I was shocked too when I learned this after all my Lux University training told me otherwise.

If your RX was ground on such a flat lens that wasn't aspheric on the front, your off axis vision would suffer from severe spherical distortion. The aspherics are able to correct those errors and therefore allow you to see out of such a flat lens for your prescription. Like hayde said say your spherical is a 5 base and the aspheric is clocking in around a 3 base. To see the benefit of the aspherics in your RX you would have to compare vision in a spherical 3 base and an aspheric 3 base, in that comparison the vision out of the aspheric lens would be markedly better.
However comparing a spherical 5 base to an aspheric 3 base - the vision at best will be equal but will never be "better".


However, long time wearers of aspherics often get "used" to the vision out of the flatter lens they get, and will feel weird if put back into a spherical. Around 95% will get used to either though in my experience. (I'm totally guessing on the percentage just my impression)

I heard back in the day there were molded Aspheric lenses that were made with better vision and not flatter thinner lenses in mind, that was before my time though. Because of the customization of digital backside mapped processing and custom geometry its the best way to go (edit: if the frame and/or RX calls for it of course. regular aspherics/spherical lenses get the job done plenty also)


Also remember if the Aspheric is fit too high or too low or too far from the face etc, all of its "benefits" will be lessened because it is assuming the eye's rotation and if the OC is placed in the wrong place (Martin's rule of tilt) the benefits won't be there, it may be your case. or you may just be sensitive.

[Prentice Pro 9000]

It's important to remember when we're talking about distortion in an aspheric lens, the basis of comparison is against a spherical lens in the same Wrong base curve. In that case, the aspheric (traditionally surfaced) will provide a superior field of vision with the correct power.

In terms of patient adaption, you'll be hard pressed to find a patient who finds the vision inferior to a conventionally surfaced spherical lens in the proper base curve, even if you detect a differential in the margins under a lensometer.

Welcome to Optiboard, Kisselt!

So, to be clear, aspheric designs lessen distortion, but are not going to have the edge to edge power that a best-form spherical lens will have?

[Robert Martellaro]

Hello, I am an optician apprentice and lab technician in lenscrafters, (at least until they close all the labs soon) and I have a question about aspheric lenses. I am a hyperope, so I'm sure you all know I have a plus prescription. Irecently had a new pair of glasses made with aspheric lenses, and I am used to spheric lenses, however I notice a discernible lessening of power in the periphery. I am able to accommodate to counter it, but I was under the impression that aspheric lenses are supposed to actually have a more uniform power across the entire lens.

Greetings. You're not accommodating to a change in power, instead it's likely that you're experiencing less magnification. That's one of the advantages of using flatter base curves for hyperopes. The brain adapts to these changes in about 10 to 20 days.

However, using a flatter than "best form" base curve will introduce aberrations, specifically oblique astigmatism and defocus (power error). For example, a cr39 +4.00 D lens has a best form base curve of about +9.00. If the BC is decreased to +6.00, the oblique astigmatism increases to .50 D, and the power error to about +.50 D, 15mm away from the primary gaze. Changing the surface design from spherical to aspherical decreases the power off-axis to compensate.

http://64.50.176.246/cecourse.php?url=lens_design/

Hope this helps,

Robert Martellaro

[Prentice Pro 9000]

Greetings. You're not accommodating to a change in power, instead it's likely that you're experiencing less magnification. That's one of the advantages of using flatter base curves for hyperopes. The brain adapts to these changes in about 10 to 20 days.

However, using a flatter than "best form" base curve will introduce aberrations, specifically oblique astigmatism and defocus (power error). For example, a cr39 +4.00 D lens has a best form base curve of about +9.00. If the BC is decreased to +6.00, the oblique astigmatism increases to .50 D, and the power error to about +.50 D, 15mm away from the primary gaze. Changing the surface design from spherical to aspherical decreases the power off-axis to compensate.

http://64.50.176.246/cecourse.php?url=lens_design/

Hope this helps,

Robert Martellaro

Do you know the actual off-center power calculation or where I can find it?

[Robert Martellaro]

Do you know the actual off-center power calculation or where I can find it?

Go to: http://64.50.176.246/tools/downloads.php

School down to Spectacle Optics, download and install, open Optical Analysis.

Best regards,

Robert Martellaro

[Hayde]

So, to be clear, aspheric designs lessen distortion, but are not going to have the edge to edge power that a best-form spherical lens will have?

Good question. Depending on the lens cut, you're probably correct: as distance from OC increases, the outer rim of the aspheric lens will approach the spherical version on the 'wrong' flat base curve. Practically speaking, it really depends on how finite a unit of measurement we're using to define power [distortion.] Either way, 'Measurable by you and me' and 'Detectable by the patient' are very different things in this case.

I would slightly revise your sentence to say "traditional (non-digital) aspheric surfacing lessens distortion [of a given lens power on a flatter-than-optimal base curve,] but are not going to mirror the edge-to-edge power curve that a best-form spherical lens will have. [Fortunately, the typical field of vision of a worn lens combined with the typical patient's inclination to turn their neck for focus virtually eliminates any prescriptive concern of that difference in the extreme periphery, although good opticians are mindful of the nuance in extremely rare cases when it's relevant.]"

...and when I say 'rare,' I'm still waiting on such an exception to present itself to me first-hand...anyone ever see this present in person?

[Robert Martellaro]

So, to be clear, aspheric designs lessen distortion, but are not going to have the edge to edge power that a best-form spherical lens will have?

Distortion, that is pincushion or barrel, is not affected by the use of aspheric surfaces for Rx ophthalmic lenses. Depending on the number of base curves available, spherical and aspherical surface designs should provide similar performance on and off-axis when fit properly. Reduced magnification or minification, lens thickness, plate height, and lens weight are other advantages of flatter, aspheric surface designs.

Hope this helps,

Robert Martellaro

[Prentice Pro 9000]

Distortion, that is pincushion or barrel, is not affected by the use of aspheric surfaces for Rx ophthalmic lenses. Depending on the number of base curves available, spherical and aspherical surface designs should provide similar performance on and off-axis when fit properly. Reduced magnification or minification, lens thickness, plate height, and lens weight are other advantages of flatter, aspheric surface designs.

Hope this helps,

Robert Martellaro

do aspheric lenses follow best-curve the same as spherical? Or would it be something like a +4.00 should be ordered in +9.00 spherical and a +6.00 aspherical for optimal performance?

[drk]

Great, confusing discussion!

Bottom line, in layman's terms?

1. "Traditional" front=molded/back=surfaced aspheric SV: Good for cosmetic/comfort. Optics are comparable to "regular (best curve) SV".

2. "Free-form/individualized/digital/custom" "apheric/atoric" SV: Good for cosmetics/comfort and optics are superior to "regular (best curve) SV".


Bottom, bottom line:
Always use FFSV. Not expensive. Awesome lenses.


What brands do we like?
Unity?
Spectrum?
Auto 2/3?
What else?

[Hayde]

Great, confusing discussion!

Bottom line, in layman's terms?

1. "Traditional" front=molded/back=surfaced aspheric SV: Good for cosmetic/comfort. Optics are comparable to "regular (best curve) SV".

2. "Free-form/individualized/digital/custom" "apheric/atoric" SV: Good for cosmetics/comfort and optics are superior to "regular (best curve) SV".


Bottom, bottom line:
Always use FFSV. Not expensive. Awesome lenses.


What brands do we like?
Unity?
Spectrum?
Auto 2/3?
What else?

I'm not sure I concede your working assumption that FFSV are relatively inexpensive. The price distinction on my formulary is significant to my SV patient base, and the optics improvement doesn't surface south of 2 diopters for a traditional aspheric vs spherical (referring to magnification/minification Robert mentioned,) 3-4 diopters for a digital vs traditional aspheric.

Since that's the case for me, I don't move as many FFSVs. The Essilor gets great results for me--perhaps better than the Autograph, but to be fair I haven't had a statistically significant number of SV Autographs to make a fair judgment yet. (My % of SV patients is about 1/2 the industry average, so there are people out there way smarter than me on this subject...and most others come to think of it....)

[Robert Martellaro]

do aspheric lenses follow best-curve the same as spherical? Or would it be something like a +4.00 should be ordered in +9.00 spherical and a +6.00 aspherical for optimal performance?

Jaketull,

The front to back curve relationship is important for both designs, more so with aspherics. The actual numbers will depend on the type and degree of asphericity, and the number of base curves available. The manufacturers provide charts that show what Rxs are ideal for a specific base curve. Using the previous example, if the +4.00 Rx is placed on an aspheric +5.00 BC, but the manufacturer recommends a BC of +7.00, the result will likely be poorer optically than if we had used a spherical +7.00 BC with that same +4.00 Rx.

Now that the lens geometry can be optimized in real-time with free-form manufacturing, the chances for bad outcomes due to unwise substitutes, i.e. to save money, time, or from carelessness, are reduced significantly, as long as the lens is positioned optimally.

In both cases (semifinished or free-form) it's recommended that we do not specify base curves, letting the manufacturer's ray tracing and computer calculations determine the correct base curve value for the best possible optics.

Best regards,

Robert Martellaro

[Tallboy]

In both cases (semifinished or free-form) it's recommended that we do not specify base curves, letting the manufacturer's ray tracing and computer calculations determine the correct base curve value for the best possible optics.

Robert this brings up an interesting question I've had for a while. When I need to go cosmetics above all else with a high plus I usually will go for a frontside molded aspheric. Since they cost as much or more than many custom digitally real time designed backside aspherics - could I expect the wearer to experience similarly successful optics if I ask for a flatter base curve on a "digital" lens instead of using a frontside molded one? If so do you or anyone else know what designs are successful for this?

I know I get insanely flat lenses from Zeiss individual sv 1.74, I'm not certain its the same as their 1.67 though. Seiko has a lens like this right? What about Shamir or IOT - do you ever request flatter base curves in those? I only ever request a base curve for sunglasses, otherwise I let them just do what the program says.

I hope I explained what I was asking about alright.

[Hayde]

Robert this brings up an interesting question I've had for a while. When I need to go cosmetics above all else with a high plus I usually will go for a frontside molded aspheric. Since they cost as much or more than many custom digitally real time designed backside aspherics - could I expect the wearer to experience similarly successful optics if I ask for a flatter base curve on a "digital" lens instead of using a frontside molded one? If so do you or anyone else know what designs are successful for this?

I know I get insanely flat lenses from Zeiss individual sv 1.74, I'm not certain its the same as their 1.67 though. Seiko has a lens like this right? What about Shamir or IOT - do you ever request flatter base curves in those? I only ever request a base curve for sunglasses, otherwise I let them just do what the program says.

I hope I explained what I was asking about alright.

While I'm waiting with popcorn for Robert's illumination along with you, I'll bring up the fact that it's quite often the case minimum edge thickness here can wipe out any significant thickness savings from a 1.74 over a 1.67. I make a point of calculating estimates for center thickness/edge thickness differentials before recommending the 1.74--once a patient selects a frame and I can take measurements to pull it off. For aspherics that thickness difference is even less than the spherical differential turns out to be.

[Tallboy]

While I'm waiting with popcorn for Robert's illumination along with you, I'll bring up the fact that it's quite often the case minimum edge thickness here can wipe out any significant thickness savings from a 1.74 over a 1.67. I make a point of calculating estimates for center thickness/edge thickness differentials before recommending the 1.74--once a patient selects a frame and I can take measurements to pull it off. For aspherics that thickness difference is even less than the spherical differential turns out to be.

I 100% agree with you (not to mention the seemingly never ending coating defects on 1.74). The Zeiss 1.74 individual (SV/PAL) is the ONLY 1.74 I will use anymore (and pretty much the only reason I use a Zeiss lens also). I absolutely don't think what they do in Germany or Japan with those lenses is the same process as any other of their Individual lenses. There has to be something else going on with these lenses. I've gotten them where the back curve seemed almost flat before, but people rave about the vision.

I offer it when the RX is over +8.00 or -10.00 or so and it is much more than the next step down - the people that have gotten them absolutely love them though, and knock on wood, no coating defects to speak of (can't say that for Crizal or any other coating.)

[Robert Martellaro]

Robert this brings up an interesting question I've had for a while. When I need to go cosmetics above all else with a high plus I usually will go for a frontside molded aspheric.

It makes sense to work the steepest curve, that's where the meat is! So for hyperopes, one might lean toward front aspheric/atorics, with myopes getting their backsides massaged. But there are so many surface design, material, and Rx limitations, compromises abound.

Note: Zeiss has a 4mm distance from the fitting point to the prism reference point...a problem for anisometropes, unless you Rx canceling prism, something that free-form optics do poorly, especially over 4 PD or so. These reference points are coincident with Shamir's SV, and is what I use when vertical imbalance is a concern.

Best regards,

Robert Martellaro

[Tallboy]

It makes sense to work the steepest curve, that's where the meat is! So for hyperopes, one might lean toward front aspheric/atorics, with myopes getting their backsides massaged. But there are so many surface design, material, and Rx limitations, compromises abound.

Note: Zeiss has a 4mm distance from the fitting point to the prism reference point...a problem for anisometropes, unless you Rx canceling prism, something that free-form optics do poorly, especially over 4 PD or so. These reference points are coincident with Shamir's SV, and is what I use when vertical imbalance is a concern.

That makes a ton of sense (where the meat is I mean). I've heard that Seiko has a front and backside aspheric/atoric lens, I wonder if the Zeiss 1.74s I get from Germany/Japan that are so crazy flat but seem to work are utilizing that tech.

Thanks for the 4mm to the prism reference point info, I always thought the PRP location didn't matter - but I guess I see if there was a major imbalance there could be issues. I almost exclusively use IOT these days for digital sv, some shamir also - both are OC on the PRP. Whatever works.

I would love Barry Santini to chime in on this thread because I know he has shown an affinity for Zeiss over Shamir digital lenses- and one of his sticking points was the location of the OC on the PRP instead of 4mm above like Zeiss.

[drk]

I'm not sure I concede your working assumption that FFSV are relatively inexpensive. The price distinction on my formulary is significant to my SV patient base, and the optics improvement doesn't surface south of 2 diopters for a traditional aspheric vs spherical (referring to magnification/minification Robert mentioned,) 3-4 diopters for a digital vs traditional aspheric.

Since that's the case for me, I don't move as many FFSVs. The Essilor gets great results for me--perhaps better than the Autograph, but to be fair I haven't had a statistically significant number of SV Autographs to make a fair judgment yet. (My % of SV patients is about 1/2 the industry average, so there are people out there way smarter than me on this subject...and most others come to think of it....)

You're right. I should have said "When considering an aspheric for cosmetics, always go FFSV".

Likewise, I'm now saying "When you have a moderately astigmatic or higher-powered lens, try to go FFSV, for better optics."

But maybe we should get to the point when we consider SV lenses totally differently, as you thought I was saying!

What if...

...similarly to the way we see a presbyopic Rx and immediately think of our favorite PAL design (only defaulting to segmented if PALs are contraindicated)...

...we looked at a SV Rx and immediately think of our favorite FFSV brand (only defaulting to a spherical lens if FFSV is overkill or cost-prohibitive)?

[Tallboy]

Because I don't surface lenses at my shop only finish, if I'm custom surfacing a lens 99 times out of 100 I end up making it a ffsv, its not that much more if at all. Front side aspheric surfaced? Often more expensive (these days) in my experience

[MakeOptics]

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