# Optical Forums > Ophthalmic Optics >  Minus aspheric lenses...please answer this!

## drk

Ok, I have a question about minus aspherics:  

1. I assume that the primary purpose for aspherizing SV minus lenses is for edge thickness reduction, and not to compensate for a flattening effect, as with plus lenses.

2. This takes the lens off "corrected curve" at the periphery, I assume(although I realize corrected curve and stock semifinished lenses is a rather rough optimization at best).

3. Therefore, are peripheral optics actually worse than non-aspherics?

Muchas gracias.

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## lensgrinder

Actually the front curve for a minus lens will get steeper as you move away from the center.  This helps to prevent the gain of minus power that the wearer experiences when they move off center.Corrected curves are only used on front spherical surfaces.The optics, periphally, through an ashperic will be better than a non-ashperic because the curves on the front somewhat change to account for the change the wearer experiences when they look off-axis.

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## HarryChiling

no longer playing in this sand box

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## drk

> Actually the front curve for a minus lens will get steeper as you move away from the center. This helps to prevent the gain of minus power that the wearer experiences when they move off center.Corrected curves are only used on front spherical surfaces.The optics, periphally, through an ashperic will be better than a non-ashperic because the curves on the front somewhat change to account for the change the wearer experiences when they look off-axis.


1. Right, but there is not more "minus power" as you "go off center", I really don't think.
2. Right, but I'm under the impression that asphericity does not improve optics over and above corrected curve.
3. Please elaborate. How can you be better than "corrected curve"?

http://www.laramyk.com/learn/aspherics_1.html
Let me include a link to some of Darryl's stuff^

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## mike.elmes

As one looks peripherally in any lens, the increase in vertex distance actually creates less power in a minus lens, more power in a plus lens. Lets use an example of a -8.00 sphere

A _spherical_ lens would be _roughly_ a +2 front base and -10.00 rear curve.

An _aspheric_ lens would be a +2 base curve in the center of the lens and as you slide the lens clock peripherally...it would increase to say a +3.00 curve at the outer edge.So the lens then has a -7.00 in the periphery. I personally have not found many myopes like s/v aspheric...at least at first. The plus lenses have a flatter curve at the periphery which reduces plus which offsets the increase in power one gets as the vertex increases. I find hyperopes appreciate the aspheric lenses.....I'm not sure why the minus lenses don't work the same way. Who has some theory on the logic of less minus in the periphery of an aspheric minus power lens?

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## OPTIDONN

Here is where I have an issue with semi-finished aspherics. The amount of asphericity is determined by the rx. Lensgrinder mentioned that for plus power aspherics the curvature flattens towards the periphery. Aspherics for minus lenses will actually steepen. Well for some base curves, 6.00 BC for example, can be used for both plus and minus powers.  :Confused:   So what type of aspheric is it?

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## mike.elmes

As quoted in Darryl's article on Aspheric lenses:

"By producing a highly-aspherical surface with a rapid change in curvature towards the periphery of the lens, the sag of the surface can be made much shallower. _When asphericity is ‘exaggerated’ in this fashion, the off-axis optics are generally poor_—since the lens has been optimized solely for cosmetics, with little regard for optics. Most modern aspheric lenses are designed to provide good optical performance, nonetheless."

I have dispensed some pretty expensive aspheric lenses to myopes who didn't like the peripheral vision, *and had to redo in spherical*.... :cry:

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## mike.elmes

good point about the semifinished aspheric lenses.....a predetermained degree of asphericity without a known finished power.

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## lensgrinder

> 1. Right, but there is not more "minus power" as you "go off center", I really don't think.
> 2. Right, but I'm under the impression that asphericity does not improve optics over and above corrected curve.
> 3. Please elaborate. How can you be better than "corrected curve"?


No, I should have said gain in oblique astigmatism.No, it does not.Well my statement was wrong.  You can be better than corrected curve when it comes to thickness, but you cannot be better when it comes to optics, you can be the same.I use Darryl's Optics Lite program and I mis-read the information.  Sorry

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## drk

Well, I do appreciate your input.  I have learned a lot from everyone on this board, and I'm still not getting everything right!

What's your opinion, at least?  Can they thin the lenses peripherally and still not lose optics?  Can they improve peripheral optics?  

From experience, I have to guess that they somehow can thin edges and still leave the optics intact.

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## Robert Martellaro

> Here is where I have an issue with semi-finished aspherics. The amount of asphericity is determined by the rx. Lensgrinder mentioned that for plus power aspherics the curvature flattens towards the periphery. Aspherics for minus lenses will actually steepen. Well for some base curves, 6.00 BC for example, can be used for both plus and minus powers.  So what type of aspheric is it?


There's actually a narrow range of powers that would be appropriate for an aspheric +6 BC SV CR39 lens, let's say +3.00 to +5.00. If the lens didn't have a flatter, aspheric BC, then the range would be about -2.00 to +2.50.

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## drk

Hi Robert.

I am perplexed by Optidonn's question as well.  There simply must be a minus aspheric series and a plus aspheric series, and you have to pull the correct one?

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## lensgrinder

> What's your opinion, at least? Can they thin the lenses peripherally and still not lose optics? Can they improve peripheral optics? 
> 
> From experience, I have to guess that they somehow can thin edges and still leave the optics intact.


I think they can with, like Harry said, using "asphericity" on the back side.  Kind of like the reverse of what they do with plus.  With "Free-Form" becoming more popular in the US you might see it a little bit more.

Also they do have a range for finished ashperics:
For example -1.00 to +1.00 will be on a 4.25 base, but that is the only time they will remain on the same base and I am sure they pick some surface that will benifit both plus and minus because the gain of oblique astigmatism in those low powers is minimal.

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## OPTIDONN

The biggest benefit of an aspheric lens is that you can create a thin lens and still maintain fairly good off axis performance. So in this base curve thats used for a narrow range of RX's, both plus and minus, you could in fact end of with a thicker lens. For example if this chosen base curve uses a form of asphericity that steepens toward the periphery and used this for a plus power lens, it could in fact end up thicker than desired. Of for a minus lens used in an aspheric form that flattens toward the periphery it would be thicker than desired. Now this thickness really would not amount to much and may in fact be thinner than a best form lens but it just goes to show that semi finished aspherics may not be very precise. However in the past I have put all sorts of prescriptions in a variety of aspherics with great results. In the end I think it all just depends on the patients tolerence.

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## mike.elmes

Long, long ago when cataract jobs were an every day dispensing job we had several aspheric choices. One that comes to mind was the Welsh 4 drop. This was a lens which had 4 diopters of asphericity or drop in base curve as measured. It was designed for a preset lens power, as 4 diopters was perfect for say a +13.50 or so. Trouble was..... cataract Rx's were all over the place for power, some as low as +9, some as high as +15. Enter the Hyper Aspheric, which was aspheric, but depended on power. Higher power more aspheric, lower power, less aspheric. This was especially helpful to improve the field of vision ,and to reduce thickness(in some areas). This concept was introduced in the aspheric lenticular series and is still available today. This was a technology that was invented to improve vision, and reduce thickness.
  Cosmolit lenses were a design that seemed to evolve from this same technology. They were a definite improvement to hyperopes. I'm really not so sure the aspheric lenses for myopes have done the same thing. First the base curves started to flatten which was offset by aspherics to reduce radial astigmatism. The nikon i for example reccomends a 3 base for a lens power up to +2.0....but is so aspheric it can give the wearer decent optics....might be a little much me thinks....but thats what the white papers say in japanese trials. The Seiko MX and SSV lenses were introduced to reduce thickness and improve peripheral vision. They use a dual aspheric design with the ssv, both front and back.

Anyone using this lens lately?

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## drk

I'm thinking that we have this to look forward to, as well, with "free-form" (3-axis generated, digital, or whatever you call it) technology:

-First, customized progressive parameters, like inset, panto, wrap

-Next, customized progressive designs by add power and distance Rx (remember, Sola?)

-Next, dual-sided, or better, back side progressive curves

-Next, perhaps "flavors" of progressive designs to be specified, depending on individual needs (sort of Sheedy's belaboured point)

And then, 

-Atoric surfaces for cylinder patients on SV and progressives
-Aspherics on front and/or back surface for SV patients

Throw in a smattering of wavefront hoopla, and you get essentially everyone maximized vision in a cosmetically good lens.  (Assuming, of course, that you choose a nice material: poor abbe could trump all these advances, perhaps).

My brain is ready to explode, now. :Eek:

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## Robert Martellaro

> Hi Robert.
> 
> There simply must be a minus aspheric series and a plus aspheric series, and you have to pull the correct one?


Yes. (Nice to see you back at Optiboard!) 

Following up on what's been said already, the lab must closely follow the manufactuers BC to Rx recommendation. For instance, Hoya's aspheric sfsv trivex comes in these BCs-.70 1.00 1.25 1.75 2.25 3.00 3.25 3.75 etc. If Hoya says a -5.00D Rx needs to be on the 1.75 base then that's what must be used to keep the off-axis power error and oblique astigmatism within an acceptable range. And with so many base curves available one might even be able to say that some aspheric lenses would have better off-axis performance than spherical lenses, where the BC selection is usually more limited.

Back to your original question




> 1. I assume that the primary purpose for aspherizing SV minus lenses is for edge thickness reduction, and not to compensate for a flattening effect, as with plus lenses.


It's the flatter BC that's principally responsible for reducing the ET, minification, sag or lens height, and weight (although if you are already on a +1.00BC there's very little benefit going to +.50 or plano base). The asphericity primarily compensates for fudging the BC.

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## Darryl Meister

You guys have probably already stated most of the important points. You can flatten a Minus lens from the recommended "best form" Base curve in order to gain a slight reduction in edge thickness. You can then apply asphericity to either the front (surface steepening towards the lens periphery) or the back (surface flattening towards the lens periphery) in order to prevent a compromise in optical performance. The geometry of the aspheric surface also provides some additional thickness reduction. Aspheric lenses really do not improve optical performance compared to traditional "best form" lenses.

As the Minus power increases, and the recommended best form Base curve becomes flatter anyway, asphericity will make less of a difference in terms of edge thickness. However, the lens designer may choose to "exaggerate" asphericity in order to improve thickness at the expense of optics. Much of this is also summarized in my Lens Design Course.

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## lensgrinder

> You guys have probably already stated most of the important points.


Yes, but to hear it from you it sounds better.

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## drk

Ah-so, Darryl-san and Robert-san:

Corrected curve for crown, 1.5:
-2 to -6 on +4.50
-6 to -10.50 on +2.50
>-10.50 on +0.50

You are saying that, say, on a -7.00 D lens

Instead of using (roughly): +2.50 front and -9.50 back curve
you can use:+1.00 front and -8.00 back curve
getting resultant edge thickness reduction, without getting poorer optics! 

Very good....


And you are saying that over -10.00, asphericity is moot, since it's already flat. (In other words, these 1.74 hyper-duper index lenses need not be aspheric).

Superb! I can sleep tonight.

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## drk

Parenthetical note: 

"Please pass the A-R: I'm wearing a plano mirror for a front surface"--Mr. Myope

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## Robert Martellaro

> And you are saying that over -10.00, asphericity is moot, since it's already flat. (In other words, these 1.74 hyper-duper index lenses need not be aspheric).
> 
> Superb! I can sleep tonight.


Yes, but keep one eye open.:)

If the -10.00 was a low index material you'll already be on a pretty flat base, but a 1.74 spherical will have a steeper best form base curve, probably enough to warrant going flatter/aspheric, at least at that power, considering that if you're using 1.74 then the goal is optimum thickness reduction. 

Regards,

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## Darryl Meister

A -10.00 has oblique astigmatism eliminated at a viewing angle of 30 degrees with a front curve of around 1.75 D. (You can verify this for yourselves with my Optical Analysis program available for download from OptiBoard.)

The edge thickness in this case at a 55 mm diamter is 10.8 mm. Now, changing the front curve to 0.50 D reduces the edge thickness to 10.4 mm (a 0.4 mm reduction). However, the oblique astigmatism goes from nearly zero to 0.36 D. You could then aspherize this lens in order to reduce the edge thickness to 10.2 mm or less, while maintaining close to zero oblique astigmatism at 30 degrees (though, depending upon the aspheric design, you may have more oblique astigmatism at greater viewing angles than you would with a spherical 0.50 D front curve).

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## EyeFitWell

> And you are saying that over -10.00, asphericity is moot, since it's already flat. (In other words, these 1.74 hyper-duper index lenses need not be aspheric).
> 
> Superb! I can sleep tonight.


Actually, while reading this thread, I was wondering if the 1.74 (essilor) is aspheric.  I had assumed it was, after having mistakenly ordered a few high index lenses that were not aspheric and noticing a thicker edge than when they were ordered as aspheric.  
Anyone know if the fabulous new 1.74 is aspheric?
For the record, I'm about -6.00 and that's what I'm wearing, and I've never had better vision, even to the edge of my drill-mounted lenses!

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## Fezz

As far as I know, the 1.74 is only available as an aspheric.  You may want to look into the Hoya 1.70. It is aspheric and many times(because of BC, RX, etc) it is actually thinner than the 1.74.

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