# Optical Forums > Ophthalmic Optics >  Which kind of lens has better quality?

## Ben

Hi, everyone, I have some questions but after reading some articles, I still can't understand. Would you like to answer my questions?

Some people say that the crown glass has the best optic quality, but the abbe value of CR-39 is also 58. Comparing Zeiss Punktal 1.5 and Clarlet 1.5, both with the same coat, except for their physical differences, are they really opically the same?

Comparing to the best form lenses, the front side aspheric lenses provide the same opitcal quality, right?

Bi-aspheric or atoric lenses provide better optics for the people have the cylinder power. Why can they do better? Is it because they eliminate or minimize the oblique astigmatic error due to the cylider power?

Some manufacturers claim that their bi-aspheric or atoric lenses provide better optics for the people DON'T have cylinder power than front side aspheric lenses do. e.g. hoya Nulux ep and AO s-lite. If it is true, how could they do that? Do they reduce more transverse chromatic aberration, curvature error, or distortion than the best form design and the front side aspheric lenses do?

Thanks a lot, 
Ben

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## chip anderson

Lens with best optical quality: *GLASS*

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## QDO1

All dispensing is a comprimise.  In the end we just choose the best comprimise for the patient, taking into account (in no particular order) frame choice, RX, environment for usage, age of patient, required coatings and tints, previous material, required basecurves, required Asphericity / Atoricity etc.

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## Darryl Meister

Optically speaking, there is really no difference between CR-39 (plastic) and glass, assuming that both are free from unwanted color and other internal defects. However, glass is frequently considered to be a more "robust" material during the surfacing process, because of higher heat deflection temperatures and such, and consequently less prone to processing aberrations. This was particularly true during the early days of plastic lenses, since the surfacing equipment at the time was really engineered for glass lens materials. However, modern surfacing equipment has all but bridged the gap in final product quality between glass and plastic lenses.

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## chip anderson

Darryl: 

 In all these years this is the first answer you have given that is full of stuff. Git youself a microscope and look at the surface of (a very high quality) glass and compare it to the best quality ever achieved in plastic.  It ain't no contest.  And this is when it's new an has not been exposed to the elements.
It's no accident that good quality optical equipment, telescopes, binoculars, microscopes, etc. have *glass* lenses.  Git youself a good quality pair of eyeglasses made from *glass* and another made "identically" from any other material, wear each for a day and then get back to me.

Chip

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## Darryl Meister

Chip, I agree with you to an extent -- but I don't know that what you're describing is really the initial "optical" quality. I think both lens materials can take a very high polish with surface imperfections that are well below what you might consider "optically visible" (that is, producing scatter and stuff). And both glass and plastic can suffer from greyness, digs, microcracks, and other surface imperfections. At least I've never seen, read, or heard anything to the contrary, beyond anecdotal claims. But, to your point, Yes, plastics will often degrade more quickly than glass, particularly after exposure to abrasives, ultraviolet radiation, and other environmental factors.

However, this is probably a bigger consideration for optical _instruments_, which need to last for years and years. Eyeglass lenses, on the other hand, are replaced every two to three years. Of course, plastics can be coated to improve scratch resistance considerably, but this generally requires an additional manufacturing process. Also, glass has been available in a wider variety of indices and Abbe values, which is useful for making the achromatic doublets used in many precision optical devices. Plastics, on the other hand, are very lightweight and very impact-resistant, which is ultimately a bigger consideration for a spectacle wearer -- at least in my opinion.

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## chip anderson

Darrrrrrl:  


I don't think you can concider lightweight or impact resistance as an "optical quality" only the ability to transmit and focus light can be concidered an optical  quality.  Here glass has it hands down.   As to gray (unpolished areas) and other surface imperfections you may have a small point here as the quality of glass surfaceing sho ain't what it used to be (which is why I put in a quality qualifier).  Back in the days of Modern Optics and the Modern Arc you couldn't have made such statements even if plastic had reached it's present stage of eveolution.
Chip

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## spartus

> Back in the days of Modern Optics and the Modern Arc you couldn't have made such statements even if plastic had reached it's present stage of eveolution.


And back before the Internet, you'd be mailing these responses back and forth to one another. To be more precise would be to say that _to the eyewear consumer_, there is no optical difference between glass and plastic. Unless you're dispensing head-mounted microscopes.

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## Darryl Meister

> I don't think you can concider lightweight or impact resistance as an "optical quality" only the ability to transmit and focus light can be concidered an optical quality.


Nah, I was just suggesting that there are other factors that may make plastic a better alternative for spectacle lenses, assuming that the optical quality is sufficient.




> And back before the Internet, you'd be mailing these responses back and forth to one another.


God bless the Department of Defense and ARPA. ;)

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## chip anderson

"A moulded lens (surface) will never be the equal of a ground lens (surface)."

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## Ben

Thanks. But I still wonder that how atoric or bi-aspheric designs improve the optics for the people have no cylider power. 

By the way, are the Zeiss Punktal 1.5 and Punktal 1.6 still made by the best form design (a point-focal lens)?

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## AWTECH

Chip said:




> "A moulded lens (surface) will never be the equal of a ground lens (surface)."


This is correct in general, however the surface quality of non glass lenses varies much more than surfaces of a glass lens.  This is due to many reasons, the flexability of the materials the various hard coatings required and this is just with cast lenses.  Injected material such as polycarbonate can start out with the same high quality raw product and then if you try to maximize production you maximize lens aberations.  Almost every optical management is pushing for the lowest price for this or that when in fact they are not comparing apples to apples.  A polycarbonate lens is very different from one injection facility to another and on top of this some lens companies use different manufactures for their poly products and package them in the same envelopes.

As Chip points out differences do exist.  A ground surface whether it is glass on a non-glass material can also vary from lab to lab surfacing the same blank.  
How many lenses are cut today using equipment with true diamond tools with precision controlled air bearings? (less than 1%)  Today, our company is able to produce a lens that can be worn without any further treatment other than the precision surfacing.  This is still under final development but it is possible to not have to fine and polish as lens like most labs produce.

The lens quality which is the original question has a lot to do with all aspects of the production process, whether surfaced by a lab or a finished lens that you purchase.

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## mirage2k2

> "A moulded lens (surface) will never be the equal of a ground lens (surface)."


This confuses me  :Confused:  
Whether a lense is moulded or ground there is always at least one side that is moulded right?  Don't blanks that have a ground back surface have a moulded front surface ... unless the whole thing is ground with a free-form generator?

Why is a moulded lens surface worse than a ground one?  You would imagine that with less robust plastics, those that are "stressed" during processing, the moulded surface would be better since it was never subjected to such stress  :Confused:

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## AWTECH

The reality of the manufactured lenses is that due to cost typically a molded or injected lens will not have as controlled surface as a ground or generated surface.  This is due to issues such as shrinkage, injected pressure, curing time and temperature control etc.  Higher volume production usually is equal to lower controls over quality.

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## mirage2k2

thanks,

I was thinking that it might have something to do with bulk processing and quick turnaround, etc.

Even so, don't the majority of lens blanks have a moulded front surface?

If a free-form generator is used to cut the lense are both surfaces ground?  From the information I've found (there isn't much out there) a lab can get the raw lens material and just require a points file which contains the particular lens design ... the manufacturer supplies this instead of blanks?

It sounds like you are designing some kind of free-form generator ... need any help with software?

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## chip anderson

A couple of points:

1) I stated *Glass*.

2) Glass lenses in past were ground on all surfaces including the fused ones. Note the comment in someone's post above: "Due to cost and efficency _aren't the front surfaces of all lenses molded?"_ Cost and efficency never have anything to do with making the "best" of anything.


3) One didn't have to have a generator, much less a "free from" generator to grind any of the surfaces of glass (except possibly PAL's which are never going to qualify as the _optically best_ lens anyway.) Lenses can be hand ground, generators are just a more efficent way of grinding.

3) As I understand this (and I have done *no* investigation into this) free~form genterators are to accomplish customised surfaces for different needs and results, not necessiarily to reach the best results for single use (optimal single function lens). i.e. customized "drop to seg" or "minimal peripheral aberration areas. I don't know that they would make a better binocular or telescope lens.

4) Plastic (most of them except probably poly) can be ground, this is just not the most efficent method of doing this.


5) Does anyone ever check plastic lenses for Newton's Rings? I don't think they can be made well enough for the test to even be done. Anyone ever see a Modern Arc in a plastic or Pal lens lab?  There's a reason why not.

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## HarryChiling

> Thanks. But I still wonder that how atoric or bi-aspheric designs improve the optics for the people have no cylider power.


It is possible for an atoric or bi-aspheric lens to be optically better for a person with no cylinder correction.  If you were to take into consideration field of view of a normal individual, we are more likely to use the peripheral edges of the lens at the 0-180 degree (left to right) in comparison to the 90-270 (up and down), also the tilt on most glasses is greater in the 90-270 than the 0-180.  With atoric and bi-aspheric designs we can correct these meridians independently, therefore making a better lens.  I don't know if it is done, if the benifits would be worth it, or if the differences would be considered negligable.

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## mirage2k2

> With atoric and bi-aspheric designs we can correct these meridians independently, therefore making a better lens


but if there is no cylinder what is there to correct? ... non chromatic aberrations?  aren't they corrected in the aspheric front surface?

I thought the whole point of atoric/bi-aspheric is to provide correct rx (throughout the entire lense), in the cylinder meridian.

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## chip anderson

Unless I missunderstand aspheric single vision lenses. Thier purpose is to more nearly mimic corrective curve lenses while incorporating flatter base curves and thinner configurations. 
Followed to it's logical conclusion: A corrected curve lens gives better vision but may be more restricted as to thickess and frame width.
i.e. the oldies are the goodies optically.


Chip;)

Ammendemt:  Distortions increase with power/thickness especially in plus lenses. So where enough size/thickness is required to inherently have a good deal of peripheral distortion aspherocity may be a decided improvement in vision away from the center of the lens.

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