Lens Quality Questions

[Gandis]

Hello All

I currently work in a retail eyeglass shop in Texas, and working on getting my certification with the American Board of Opticianry. I have been reading alot, and have bought a few books through Amazon to learn more about this field.
I have a few subjects that I'm confused about that I hope more knowledgeable persons can help me with.



1. The difference between poly-carbonate and high index lenses.
Is the difference only in the thickness and weight of the lenses, or is there a difference in quality of vision from the different lenses?
For example, will a poly-carbonate lens with anti-reflective coating offer the same vision quality as a high index lens? Or will one or the other be superior in terms of vision quality?
I know hi index is more expensive and supposed to be higher quality, but I have read that many people complain of chromatic abberation from hi index lenses.
It seems most professionals on this board dislike or even despise polycarbonate?




2. What are the best eyeglass lenses in terms of vision quality. Weight and thickness aside? Vision quality, as in clarity. I know this is a general question and is variable to each patient's circumstances, but assuming single vision lenses, the way I understand it now, we have in terms of increasing quality:

Glass
CR-39
Polycarbonate
Trivex
Hi index
digital




3. Digital lenses:
How do digital lenses differ from standard lenses? Is it simply the material, the manufacturing process, or something completely different?

Who will notice a major benefit in digital lenses?
For example, will people with myopia benefit more from digitial lenses than people with hyperopia, or vice versa?
Do digital lenses offer better vision quality to people with moderate/high astigmatism?


4. I hear phrases similar to "x type of lens is better for people with mild myopia" or "x coating is best for people with high astigmatism"
Are there accepted values as to what is "high" vs "low/moderate" etc. when it comes to myopia, hyperopia, and astigmatism? Or is it case by case basis, and learned with experience and intuition?


Sorry for the long post, and I hope no one gets annoyed with all my questions.

[Chris Ryser]

Welcome to OptiBoard ..............your are in the right place to get some answers, Your post is well and nicely set up.

What you are asking will take hours to type up and you will have a hard time to get any detailed answers. So you most probably will get some partial answers.
The best way to do it is to ask one short question at a time to get the best answers.

[MikeAurelius]

2. What are the best eyeglass lenses in terms of vision quality. Weight and thickness aside? Vision quality, as in clarity. I know this is a general question and is variable to each patient's circumstances, but assuming single vision lenses, the way I understand it now, we have in terms of increasing quality:

Glass
CR-39
Polycarbonate
Trivex
Hi index
digital

Glass lenses should be at the top of the list for best optics and therefore best vision, assuming clear single vision. Digital plastic would be next. Some here will argue that glass is #1 but *shrug* it is what it is. And BTW, glass *CAN*/*COULD* be digitally surfaced, however, since it is considered an *obsolete* material by the lens makers and machine makers, you don't see it available. That doesn't mean it can't be done.

[Wes]

I know glass can be poured into a digital mold, but I didn't know it could be back-surfaced via CNC cutting. How does that work?

[MikeAurelius]

Precision optics does it all the time with single point diamonds. They use an "air spindle" (no bearings, spindle "floats" on air) at very high speed, usually around 10K RPM. That gets the required surface RPM to grind away the glass instead of pulling bits and pieces out.

[NCspecs]

Welcome to Optiboard Gandis!

Regarding your questions, here is my advice. Read your material. Use the search option here on optiboard. Really apply your critical thinking skills especially when taking information from the internet. Most of the opinions regarding different types of lenses and materials are subjective. You'll basically know more and develop your own preferences with experience.

Also, take the advice of Mr. Ryser and ask a question at a time. You'll get more indepth information. :)

[MikeAurelius]

http://www.bryanmachine.com/spindles/

[Wes]

Precision optics does it all the time with single point diamonds. They use an "air spindle" (no bearings, spindle "floats" on air) at very high speed, usually around 10K RPM. That gets the required surface RPM to grind away the glass instead of pulling bits and pieces out.

I was thinking of something along those lines. I didn't know anyone was doing it. Wax or alloy blocking?

[MikeAurelius]

Has to be alloy, there can be a heat issue, especially on thinner runs where the lens thickness is less than 1 mm. They typically use a high temp alloy, around 200 F. Synthetic coolant as well, very oily feeling.

[Gandis]

Thank you all for your responses.
I realize now that there are too many details to answer all of them.
I will try and repost smaller questions as I learn more.

[kentmitchell1961]

Glass is the best, High Density plastic is tied with cr-39, poly should not be used for humans........
as to thickness, the HD is thinest, poly next and cr39 last... the rest requires too much typing..... ;>

[mattc-ait]

If you are just looking at ABBE values, I would say Glass>CR-39>Trivex>Hi-Index>Poly. Basically, the higher the refraction index, the worse the chromatic abberation. What you choose is going to depend a lot on how much power the script needs. You may be forced into high-index at some point so your customer is not wearing hockey pucks on their face. Also, if your doing rimless that is going to rule out CR-39 as it chips and cracks easily when drilling in addition to impacts from dropping the glasses. I am not an optician nor an expert but I know a little bit about the materials. Someone else can help you fill in the blanks as many have before me.

[Gandis]

Thanks guys, I've learned a lot already reading through all the responses, and also learned that I have a looot more to learn.

From what I understand, freeform/digital/HD lenses refer basically to the manufacturing/surfacing process of the lens. Same as Hi-Index refers to using a more dense/compressed material.

As Mr. Mike Aurelius mentioned, that glass can be digitally surfaced, but it is very uncommon, because glass is considered an obsolete material.

Would a normal glass lens still offer better vision quality than a freeform plastic lens?
Would a glass lens' vision quality suffer at higher indexes?

Thanks
Gandis

[Tallboy]

good thread. Isn't Poly different though? Do the chromatic abberations that can happen in poly (well made, measured, generated and edged) that you guys often speak of occur because of its index related ABBE value? I always thought it was because of the crystaline nature of CR-39, Trivex and 1.60 that its optics "popped" not their respective ABBE values.

[Barry Santini]

I'd simplify and go with a good FFSV CR-39 or trivex lens for best vision/optics in eyewear. Least surface degradation? That'll be glass.

B

[MikeAurelius]

As Mr. Mike Aurelius mentioned, that glass can be digitally surfaced, but it is very uncommon, because glass is considered an obsolete material.

Would a normal glass lens still offer better vision quality than a freeform plastic lens?
Would a glass lens' vision quality suffer at higher indexes?

Thanks
Gandis

Yes and yes.

[Barry Santini]

Yes and yes.

Correction: NO & Yes.

B

[MikeAurelius]

Sorry, Barry, but I disagree.

[Wes]

Mike and Barry, are we comparing apples to apples?
Freeform single vision or aspheric/atoric will outperform standard spherical lenses. Visually, a FFSV CR-39 will outperform standard spherical glass, but that's apples to oranges. A fair comparison would be FFSV CR-39 vs FFSV crown glass.

[Barry Santini]

Mike and Barry, are we comparing apples to apples?
Freeform single vision or aspheric/atoric will outperform standard spherical lenses. Visually, a FFSV CR-39 will outperform standard spherical glass, but that's apples to oranges. A fair comparison would be FFSV CR-39 vs FFSV crown glass.

Mike:

This is what I'm talking about!

B

[MikeAurelius]

And, as I said before, these types of lenses CAN be made in glass, therefore, the Glass lens DOES outperform CR-39 ALWAYS.

Now, if you want to quibble on the fact that no one *IS* making glass freeforms, or the fact that you wouldn't sell glass anyway, that's a totally different discussion.

[Wes]

And, as I said before, these types of lenses CAN be made in glass, therefore, the Glass lens DOES outperform CR-39 ALWAYS.

In visual performance, marginally. Very few humans could perceive a difference, and only in extreme prescriptions. In ability to consistently surface highly accurate lenses, yes. In scratch resistance, absolutely. In weight, never. In impact resistance and cost? I don't have the data to say, but my best guess is no.

[MikeAurelius]

Wes, none of those items were part of the question by the original poster. Not that I disagree, but you are adding provisos to the question that relate to selling a product, not on how the product performs optically.

What are the best eyeglass lenses in terms of vision quality. Weight and thickness aside? Vision quality, as in clarity.

[Barry Santini]

And, as I said before, these types of lenses CAN be made in glass, therefore, the Glass lens DOES outperform CR-39 ALWAYS.

Now, if you want to quibble on the fact that no one *IS* making glass freeforms, or the fact that you wouldn't sell glass anyway, that's a totally different discussion.

In my mind, ONLY ZEiss or Rodenstock vetted FF glass would be representative of the best plastic. I think they make it in Europe (not sure).

Ok, it's glass. But by a micron!

B

[MikeAurelius]

A micron here, a millimeter there, it's still glass! Care to restate your comment to me?