Truly Aspheric?

[mrba]

I have been told that some lenses require special aspheric tools to be surfaced. Spectralite and Gradal top for instance require this.

My question is, does this indicate that a lens is "fully aspheric"? Is it possible for instance, for a panamic that doesn't require this tool to be as aspheric as Gradal?

Did I ask that well?:hammer:

Perhaps someone knows what I am trying to ask or just reads minds...

[shanbaum]

mrba said:

I have been told that some lenses require special aspheric tools to be surfaced. Spectralite and Gradal top for instance require this.

Told by whom? Darryl would probably be a more authoritative source on this, but I believe that all aspheric designs available in semi-finished form presume spherical or toric backsides.

That excludes the free-form stuff, which is at this point, in my opinion, experimental - and not available in semi-finished form.

The term "fully aspheric" evokes "fully pregnant" in my mind...

[Joann Raytar]

mrba,

I just heard the same thing; that the future of surfacing holds cutting front and back bi-aspherics in house.

I don't know how far away that is or how much the cost is going to be but it sounds expensive.

[shanbaum]

Jo, that wasn't the question, which was: is there something about certain current lens designs, such as Gradal Top, that requires aspheric back curves in order for the design to be fully functional, or fully realized.

To which, I think, the answer is "no".

Free-form surfaces, whether limited to one side or not, may well be "the future of surfacing", but that's a different question.

I might add that I rather suspect "fully aspheric" to be a marketing term, as opposed to a technical one.

[Darryl Meister]

I must admit that I'm not sure what mrba is asking here...

Spectralite is a material, not a design, so it wouldn't require anything special. Surfacing tools, for conventional machines anyway, cannot be aspheric, since you can only fine and polish spherical and circular toric surfaces with them.

He might be referring to shims used for blocking highly aspheric progressive surfaces, but these aren't particularly special -- and have been around for many years.

mrba, can you clarify a bit for us?

Best regards,
Darryl

[mrba]

I have been told by two different labs, that there is such a thing as a tool that is somehow different, and used for "truly aspheric lenses".

I have also been told that on high index/poly Gradal tops they cannot be surfaced too thin or they will lose their apsheric qualities.

I have also been told that grinding prism is preferred to decentering it on aspheric desighn lenses such as spectralite... However if you grind too much prism you will ruin the aspheric features.

Given these three things, am I reasonable to infer that the back side of an aspheric desighn lens has aspheric features as well, that are determined in part by how the lens is surfaced?


Much more focused now...

[shanbaum]

The first two assertions are simply false. Well, I suppose you could grind any lens so thin that it would lose its optical properties. But not specifically its aspheric properties.

The third is true; if prism is prescribed on an Rx, and an aspheric blank is used, the Rx prism must be ground in the lens. Decentering the lens to induce prism is verboten, as the wearer must gaze through a point near the pole of the aspheric field.

Likewise, prism cannot be ground to move the prism reference point on an aspheric lens as it can on a spherical one, as the PRP must coincide with the pole of the aspheric field.

As far as the amount of prism that can be ground in an asphere - I'd have to think about that one. Offhand, I wouldn't think you'd run into a practical limitation, even if there is a theoretical one - and I don't really think there's a theoretical one specifically related to the asphericity of the front surface.

[Darryl Meister]

Yeah, what Robert said. ;)

There are certain free-form progressives that have complex surfaces milled on the back using special machines, but these are usually only available from manufacturer-operated labs. Many manufacturers offer "free-form" lenses now, though the implementations vary considerably. Maybe your lab person is referring to these as "truly aspheric." (Strangely enough, they actually wouldn't be considered aspheric at all, since they belong to a more sophisticated class of surfaces.)

Best regards,
Darryl

[mrba]

The free form i have seen have +cyl ground in among other funky things that defy my lensometer. I will show this thread to a few other friends and try to figure out what they meant.

[Darryl Meister]

Be sure to let us know how it goes.

Best regards,
Darryl

[mrba]

A friend of mine who has been an lab owner for 500 years sent me this which helps to clairify...


..."Aspheric type lenses, progressive or single vision are much more difficult for us to grind because the curve is much flatter than normal both front and back, unusual tools are generally required. It causes thickness issues, power issues and fining marks because of the flatness. Also, because it's aspheric we are guessing at what to do on regrinds. You can't determine the base curve because it changes widely across the product front
surface. In addition to that many people try to put aspherics on people with too much decentration. All aspherics are designed for virtually no decentration so that the aspheric bubble is right in the center of the lens for best performance"...

So it would seem normal albiet flatter tools are used. And it would also seem that the specific lens desighn being fully aspheric (or fully pregnant) is determined by the front curve desighn.

So is it rigourus to determine how aspheric a grinded lens is, by observing the fact that one uses a flatter tool than a similar functioning lens all other things being equal like front curve?

[keithbenjamin]

Scott,

The tools aren't at all abnormal. As you said, generally speaking ashperic lenses will have a flatter back curve. However, we use flatter curves for many circumstances other than ashperics. For example: flatter curves = less magnification from an aesthetic point of view.

-K

[mrba]

"So it would seem normal albiet flatter tools are used."

Is what I said...

[keithbenjamin]

Yes, and if you read what I said, it says, "as you said." :D

I was just pointing out that a flatter curve does not necessarily equate to an aspheric lens. Which I think is what you were asking.

-K

[JRS]

It just appears to me that HE has trouble grinding aspherics lenses, in general - not that the back surface is aspheric.

[mrba]

K, you also said "Scott, the tools aren't at all abnormal" which is why I said that I said "So it would seem normal albiet flatter tools are used."

So we agree.

KB
"I was just pointing out that a flatter curve does not necessarily equate to an aspheric lens. Which I think is what you were asking."

Not really, when we aren't changing the curve because we want it to be gorgeous or the other unspeakable "real reason", ahem, my question still is only partially answered. I said "all other things equal" actually which means we havn't even gotten the partial answer. Beacuse I want to know if we intentionally pick a 4 base spectralite vs a 4 base comfort. Does the spectralite require a flatter tool, and does this mean that it is more aspheric?

Gentleman,
I realize I am picking a way out there, non realistic, and scientifically stupid way of approaching my question. So maybe I need to think about my question about determining degree of asphericty differently. But it would be nice off the cuff to invoke a little Occam, Ceteris Paribus of course.

[mrba]

JRS-
I don't grind lenses. In a round about sort of way Keith grinds my lenses.

[keithbenjamin]

Ok, I'll take another stab at this.

It seems you want to determine through inspection whether or not a progressive lens is "fully aspheric" or determine some degree of asphericity.

All progressive lenses I suppose can be considered "aspheric" in sense that they have non-spherical curves in the add, but not in the sense that they reduce distortion at the edges. I suspect the term "fully ashperic" is a marketing term (Darryl?) that refers to newer-generation designs where asphericity (the reducing edge distortion kind) is designed into the distance portion of the lens in addition to the "asphericity" of the near.

The degree of asphericity of a SV lens can roughly be determined by looking through the lens at an aspheric grid (or even a newspaper), moving the lens away from the grid and checking for distortion at the edges. I suppose you could do the same thing with a progressive, looking through the distance portion, although you'd have to be careful not to confuse distortion induced by the add with any "non-ashperic" distortion. You'd also probably want to use a lens that had no cyl.

[chip anderson]

Remember: The definition of aspheric is "not a sphere." Therefore anything that is not a sphere is aspheric. This includes any aberrated piece of junk.

[Darryl Meister]

My thoughts on the matter, which will reflect some of the earlier comments of others, as well:

1. We first need to clarify the rather ambiguous term "asphericity" in progressive lens design:

Since progressive lens surfaces are non-spherical, Yes, you could argue that they are all "aspheric." Referring to a progressive as "aspheric" simply because it is "not spherical" is technically inaccurate and potentially misleading, especially if the term is used to imply a more advanced level of sophistication than the progressive lens truly offers. Progressive surfaces are not really "aspheric," since the term implies that the surface has radial symmetry. Nevertheless, the term is still used in the context of progressive lenses in the following two ways:

In the "basic progressive design" sense of the word, aspheric progressives are lenses that have distributed some of their unwanted astigmatism into the distance zone in order to soften the design by reducing overall levels and gradients of unwanted astigmatism. These lenses are generally called soft designs nowadays (as opposed to hard designs).

In the "optimization" sense of the word, asphericity can be used to neutralize off-center aberrations and widen the zones of vision. This is also the kind of asphericity used for single vision aspheric lenses, and is often used when the base curves are flatter than conventional best form base curve recommendations. It is generally much more subtle than the asphericity used to distribute the unwanted astigmatism as previously described.

Note that these two uses of the term "aspheric" have nothing to do with each other. A really soft (or "aspheric") progressive lens design may or may not have optimized asphericity, just as a harder lens design may or may not have optimized asphericity. It's generally more complicated to apply optimized asphericity to any progressive lens, particularly across all of the viewing zones, but more and more progressives are starting to employ this kind of scheme.

2. Progressive lenses without optimized asphericity (whether they are soft or not) will require average back curves. Aspheric single vision lenses and progressive lenses with optimized asphericity, which generally all use relatively flat base curves, will demand slightly flatter back curves in higher plus powers. However, it is no more difficult to fine and polish the back curves of an "aspheric" lens than any other lens. The back curves are all the same, though sometimes they are obviously a little flatter.

I've heard labs complain about fining and polishing especially high back curves before, but I've never really heard any complaints about the 2.00- to 5.00-diopter back curves required for high plus aspheric lens designs, and intuitively I can't see why they would. However, if your lab has expressed difficulty with them, I won't argue the point. Modern three-axis generators shouldn't have any problems cutting flatter back curves, but older diamond-wheel generators have a practical limit of around 3.00 diotpers.

3. Thickness issues with progressive and aspheric lenses can generally be controlled with good laboratory processing software (assuming your equipment is properly calibrated and such). Most manufacturers supply surface description data for these lenses to lab software vendors, but it is up to the individual software vendors to determine how they are used. Needless to say, it requires a great deal of computation to arrive at accurate figures for the generator. Also keep in mind that the you can still ruin a job, even if the software has predicted the correct figures.

Since progressive lenses are not symmetrical, they are sensitive to the blocking process. Applying pressure at the top of the lens blank (recommended), for instance, may give you different prism and thickness results than applying pressure at the bottom. Block diameters can also have an impact. Softer ("aspheric") progressive lens designs may require a little more attention since the distance portion of the lens is even less "spherical." Optimized asphericity may or may not make any more of a difference, depending upon the amount.

4. If you are not sure whether your progressive lens has optimized asphericity or not, you could always ask. (I'd like to think that most manufacturers are forthright about this sort of thing.) You can also examine the base curve selection chart for the lens. If the recommended base curves are flatter than normal, the lens at least requires optimized asphericity. However, a lens fully optmized with asphericity for the position of wear may or may not use flatter base curves.

I think that addresses everything...

Best regards,
Darryl

[keithbenjamin]

Thanks Darryl. Excellent explanation.

Although, MRBA stated we grind his lenses, if we ground ALL his lenses he wouldn't have to deal with these other labs that have difficulty surfacing aspheric lenses. ;)

-KB

[mrba]

Darryl thank you for answering my question and helping me clairify that question.

Keith,


My labs have no current problem with aspherics. They admit they did in the past, and have learned from it. I appreciate humility from my excellent suppliers.

[keithbenjamin]

I was of course only joking thus the ";)" . I certainly meant nothing disparaging to your other suppliers. We certainly appreciate you both as a friend and a customer (at whatever level that may be).

-KB

[Darryl Meister]

Sounds like Keith just has confidence in his lab's work. I don't know that there is anything terribly wrong with that... ;)

Best regards,
Darryl

[mrba]

Kieth's lab does gorgeous work.