# Optical Forums > Ophthalmic Optics >  Center Thickness

## Samuel Jong

Is there standard center thickness (minus power) for progressive lenses?

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## HarryChiling

The standard is not nessecarily on the type of lens, but on the impact resistance.  Look into the ANSI standards.

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## Samuel Jong

> The standard is not nessecarily on the type of lens, but on the impact resistance. Look into the ANSI standards.


The custmores here don't care about the impact resistance, but instead they want their lenses thinner as possible, especially for minus power. Usually I made 2mm for index 1.498, 1.4mm for 1.54 and 1.2mm for 1.60 respectively.

I am so sorry by throwing a stupid question to you. Thanks.

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## HarryChiling

If impact resistance is not a consideration than you can make it as thin as you want.  Keep in mind that the lens base changes according to the add power and you must compensate for that.  In the US we use prism thinning to compensate for the thickness created by the progressive lens wich is usually equal to 2/3 the add power, however this is only a rule of thumb and the more accurate methods would be to take all factors into account including frame B size, power and base.  I would suggest doing a search or some research on prism thinning.

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## Samuel Jong

> Hope this helps if you need more info check out www.technicalopticians.org and do a search for "saggital" , or download all the equations.


Harry, I learned about CT and ET from the above website.

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## harry a saake

Samuel, this has been a pet peeve of mine for years, why do you want to grind a lens down to 1.2 or 1.4, what ever you take off the center is the same amount you will lose on the edge. i think all lenses should never be less then 1.8, reason being that when you grind down the lens that thin, the lens can now flex more easily, especailly in these frames today with the real narrow "B" measurements. Also, Samuel, how many of your patients who are wearing say a -4.00 could tell if they had an extra half a millimeter thickness?

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## Samuel Jong

> Samuel, this has been a pet peeve of mine for years, why do you want to grind a lens down to 1.2 or 1.4, what ever you take off the center is the same amount you will lose on the edge. i think all lenses should never be less then 1.8, reason being that when you grind down the lens that thin, the lens can now flex more easily, especailly in these frames today with the real narrow "B" measurements. Also, Samuel, how many of your patients who are wearing say a -4.00 could tell if they had an extra half a millimeter thickness?


Harry, most of the patients here always want their lenses as thin as possible at the edge, and they even don't care about the safety center thickness. I sometimes grind the SOLAOne 1.6 down to 1.1 just to meet the patient's requirement. Thanks for your advise.

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## francisOD

I am just curious about CT as well.  Can anyone else comment on the fact that if you reduce ct by 0.5mm, edge thickness will be exactly reduced by the same amount no matter what the A, B or ED is.  Also would it not stand to reason that if you have a small B, you would have more support for the lens in a full rim frame thus decreasing the chance of warpage?  Even with 1.2 ct and a small B say 18, there is support through out the periphery which should reduce risk of distortion...As an OD, this is not my strenght but this logic makes sense to me...maybe only me!!!  Looking for feedback.

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## Dave Nelson

I hope I understand your questions correctly. Imagine a plano base curve, -4.00 lens ground to 1mm center. The back surface is (obviously) a -4.00. Edge that lens into a frame with a small depth measurement, and you have thicker nasal and temporal edges than upper/lower edges. Now take a plano lens 1mm thick and using 4.00 curves, edged to the same shape, then laminate it to the back of the rx lens. You now have a lens made 1mm thicker uniformly across the whole lens, and the power remains the same. Now you can reverse that, and start with a 2mm ct lens, and grind 1mm off, and you are back where you started. Taking a mm off the center takes a mm off the edge. Regarding your second question, take about a stiff piece of cardboard, about  6 inches by three. Try bending it. You will notice it bends far more readily lengthwise that widthwise, and, as the bend is put into it lengthwise, it becomes even more difficult to bend it widthwise. Lenses are no different of course, and when plastic lenses are quite warm, as on a hot day, they are prone to warping due to uneven stresses put on them. I hope this helps.

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## francisOD

Thanks dave for a very easy to understand explanation...very basic but to the point.  Is there more chance of a lens warping in a smaller frame or a bigger frame?  In your cardboard example, it is easy to understand that the narrow B will be hard to bend (distortion) than the A, so you would seem to agree that in smaller frames, the probability of distortion is smaller.  Is this true even in myopic lenses where lens thickness increases away from the center of the lens?

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## HarryChiling

> In your cardboard example, it is easy to understand that the narrow B will be hard to bend (distortion) than the A, so you would seem to agree that in smaller frames, the probability of distortion is smaller. Is this true even in myopic lenses where lens thickness increases away from the center of the lens?


I think what you mean is that the bend occurs along the 90 degree meridian in the smaller B sizes and that holds true especially for the myopes, because of the fact that the lens is much thinner along that meridian than the 180 or the A.  I am sure everyone that has been doing this optical thing for a while has seen the small plastic frames with shallow B easurements that are bowed out of the front of the frame, this is due to flex wich can be from a combination of too large of a lens, to thin a lens, and incorrect beveling to name a few.

I have seen some lenses from overseas come in almost paper thin in the centers, as an optician I feel it is necessary to inform your clients of the risks involved with thin lenses.  There is a limit and t is an opticians job to not let the patient cross that line, after all if any patient ha a choice they would say make it as thin as they could possibly go, theoretically it could be surfaceds down to fractions of a mm, but you would lose the functionality of the lens.

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## harry a saake

Harry, perfectly said, FrancisOD, also think of this like a piece of string that you have laying out flat on a table, now curve the string, and if you do nothing more, it stays that way, but pull on both ends at the same time and what happens, the string now becomes straight, which is what exactly happens to the top and bottom of the frame and even more so with these frames with the small b measurements.

Another thought that no one ever thinks of is the fact that the bevel of the frame has a certain width to it. If you grind down the lens so thin, that it does not take up all the space of the bevel, that lens is going to wind up being loose real fast.

As HarryC. said you can only let your customers control you to a point and then you have to do what is smart for both of you. Why do you want to risk a lawsuit over a half a millimeter?

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## hipoptical

> I have seen some lenses from overseas come in almost paper thin in the centers, as an optician I feel it is necessary to inform your clients of the risks involved with thin lenses.  There is a limit and t is an opticians job to not let the patient cross that line, after all if any patient ha a choice they would say make it as thin as they could possibly go, theoretically it could be surfaceds down to fractions of a mm, but you would lose the functionality of the lens.


Risks with thin lenses? I agree if you mean problems associated with fit in particular frames. Optics should always be correct no matter the thickness. In the USA "we" are obsessed with "protecting" other people's "rights", and thus we are afraid to make lenses thin. "What happens if you get hit in the eye with a rock, Mr. Jones?" What I want to know, is what happens when people get hit in the eye when wearing no glasses at all? They should sue God for giving them perfect vision, and not having to be convinced to wear super-high-impact-bullet-proof-won't-break-light-weight-thinner-than-the other material-but not as thin as other countries-lenses by the "optician" who had a total of 3 hours "training" and will be manager soon.
I say make 'em thin! As thin as optically possible. If you need impact-resistant lenses for whatever reason, get those ALSO; for dress get 'em below 1mm CT. (By-the-way: I make 1.56 with a 1.0 CT and it passes drop ball testing at over 100 inches even after A/R.)

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## Samuel Jong

> (By-the-way: I make 1.56 with a 1.0 CT and it passes drop ball testing at over 100 inches even after A/R.)


What's the thinnest CT you had ever made for mineral 1.7 & 1.8? Thanks.

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## Ory

> What I want to know, is what happens when people get hit in the eye when wearing no glasses at all? They should sue God for giving them perfect vision, and not having to be convinced to wear super-high-impact-bullet-proof-won't-break-light-weight-thinner-than-the other material-but not as thin as other countries-lenses by the "optician" who had a total of 3 hours "training" and will be manager soon.


Except that the lens in front of the eye is much more likely to splinter into sharp fragments. Blunt trauma to the eye is bad but penetrating injury is often worse.

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## hipoptical

> What's the thinnest CT you had ever made for mineral 1.7 & 1.8? Thanks.


Don't make those... sorry.

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## hipoptical

> Except that the lens in front of the eye is much more likely to splinter into sharp fragments. Blunt trauma to the eye is bad but penetrating injury is often worse.


Yeah, that's the theory. My point is, why do we (Americans) feel the need to shoulder the responsibilities that others should carry. It should be up to the wearer to take the risk or not. They should be informed of what MIGHT happen in case by some freak accident they are hit specifically in the eye. 
What we should do is lobby to make all clothing manufacturers fabricate using only those materials that would stop sharp objects and bullets, rocks, and U.V. A, B, C and be tear resistant and come with a warranty. We all know that many more injuries are sustained in the main part of the body, so if we're forced to wear clothes, then the makers should provide us with the safest possible. I had a hat on one day to protect my head from the sun. I got hit and some of the fibers went into my skin. That wouldn't have happened if the hat was made better, so I'm gonna sue. (I'm using the lawyer that got the McDonalds coffee lady her money.)
What if you wear gas-perm contacts and get hit in the eye? Instant R.K.? Instant Lawsuit. Welcome to America... how may we sue you?
If I educate you I've fulfilled my responsibility. The rest is up to you, or should be. Of course, to educate another, one must be educated himself... and thus the real problem rears it's ugly head once again.

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## Darryl Meister

Keep in mind that in the United States we are obviously required by law to supply lenses that can withstand a certain level of impact. Consequently, given that impact resistance is proportional to the center thickness, squared, thin centers are limited first and foremost by the law (even if the wearer isn't particularly concerned about impact resistance). However, even if no such requirements were in place, the flexural stability of the lens material will still place a practical limit on thickness.

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## HarryChiling

> (By-the-way: I make 1.56 with a 1.0 CT and it passes drop ball testing at over 100 inches even after A/R.)


And do you test every pair you make this way or do you test one out of a batch?  Hey make them as thin as you like, but keep in mind this isn't like the lady sueing McDonalds, because if you get sued they are going to make you look like an optician that just disregarded the standards.  How many opticians that would be called to testify would back you up?

Let's look at in perspective:

How many of you would increase the risk of serious injury or enucleation 10%; to shave 0.2mm off of your lens?  What about 10%, for 0.4mm?

These figures are not in any way accurate and the risks are extreme, but the thickness is real.  It's not very difficult to inform and convince your client to take your advice for a safer lens.

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## Samuel Jong

Originally Posted by *Samuel Jong*
_What's the thinnest CT you had ever made for mineral 1.7 & 1.8? Thanks._




> Don't make those... sorry.


For mineral 1.8 with 1.1 CT is fine? Or what's your recommendation?

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## hipoptical

> And do you test every pair you make this way or do you test one out of a batch?


Of course not every pair is tested. I have never seen any lab test every lens they make. (Most labs don't test at all any more...) We do test, though on a regular basis, and certainly more than one lens or pair. Just because I think all the regulations are over-kill, doesn't mean that we don't follow the rules. We just found a way to produce products that are thinner, yet still over-exceed the ANSI requirements for impact-resistance. Any time something is changed in our process, or new products are developed, extensive testing is done to ensure quality on every level. (We are not in opthalmic eyewear only. We do work for the military [domestic and foreign allies],  the computer industry, the plano sunglass market, and have  done some things for the space program.  We're not a flighty organization. I just know that in America, our regulations are over-kill in many cases. We do things not because it's practical or best, but because it's the easiest way, or to avoid lawsuits. That's why the rest of the world surpasses us in many things in general, and in most aspects of the eyeglass industry.

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## hipoptical

> Originally Posted by *Samuel Jong*
> _What's the thinnest CT you had ever made for mineral 1.7 & 1.8? Thanks._
> 
> 
> 
> For mineral 1.8 with 1.1 CT is fine? Or what's your recommendation?


I cannot speak with any degree of intelligence about these lenses, as I have not worked with them. I can only say that I have seen lenses like them ground to 1.0 CT, and I was impressed with how they looked and with the clarity optically speaking. With many things, the process itself has much to do with the outcome. Two identical lenses, processed differently, could yeild different results. For a clear answer, someone else needs to respond that knows by experience, and is willing to share.

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## HarryChiling

> Of course not every pair is tested. I have never seen any lab test every lens they make. (Most labs don't test at all any more...) We do test, though on a regular basis, and certainly more than one lens or pair. Just because I think all the regulations are over-kill, doesn't mean that we don't follow the rules. We just found a way to produce products that are thinner, yet still over-exceed the ANSI requirements for impact-resistance.


 I am not saying you have a flighty operation.  Wha I meant was if you surface close to the brink of being within standards then you don't know if that one lens you tested 5 years ago with a 1mm ct and AR passed by a fluke or if it truly can withstand the test.  It's not that it is not possible for lenses to go thinner, but at a certain point you run the risk of breaking them.  For example one of the labs I use will charge extra for a poly with a 1mm center thickness and that is due to the fact that when they surface beyond their set limits they will test every pair and some times they break.  Higher breakage means higher cost.

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## Darryl Meister

> For example one of the labs I use will charge extra for a poly with a 1mm center thickness and that is due to the fact that when they surface beyond their set limits they will test every pair and some times they break.


You should probably confirm this with your laboratory. There are _currently_ no officially accepted methods for the non-destructive impact testing of plastic lenses. Consequently, impact testing for plastic lenses is _typically_ done using batch sampling. Any "breakage" when surfacing 1.0 polycarbonate probably refers to surfacing aberrations, off-powers, and so on.

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## HarryChiling

I spoke to manager at the lab, unless he is blowing smoke up my you know.  If it is due to abberations all the same it still wont meet standards, but being that I work in a pediatric practice we are very strict on everything being impact resistant.  I have parents all the time ask if we can go thinner and I can't and usually explain that the safety is not something we would consider budging on for vanitys sake.  I have actually lost customers due to thickness.

If this lens is part of a batch sampling and a some lens passes than this lens passes, but is this lens the reference lens and what is the % of defect that comes from this lens.




> Any "breakage" when surfacing 1.0 polycarbonate probably refers to surfacing aberrations, off-powers, and so on


Good point Darryl however these are considered surfacing errors when sampling and I was under the impression that they did not count the same way in the sampling.  Please correct me if I am wrong.   

The doctor I work with is lobbying within our state to make sports googles a mandatory thing on the state fields.  I have seen video and pictures of some pretty messed up eyes due to impacts.  I myself am a little more cautious with the

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## hipoptical

> For example one of the labs I use will charge extra for a poly with a 1mm center thickness and that is due to the fact that when they surface beyond their set limits they will test every pair and some times they break.  Higher breakage means higher cost.


No lab I have ever worked in or run tested every lens surfaced to 1.0 CT. I challenge labs to even be able to prove that ANY testing is done regularly. Smoke- being blown in places unmetionable....
(Just my lowly opinion, which I will hold ontil proof is shown otherwise. I've been in meetings with managers and owners throughout the nation, and testing of any kind is a joke. Everyone still talks about it- but all tongue-in-cheek, unless talking to a customer. Then, of course we all test: impact, scratch, weather, durability, you name it! I will attest that our testing is documented here for our own tests, and at COLTS for the tests they perform on our products.)
Your lab charges for 1.0 CT because they *can*, not because they have to. If they have good processes and controls, breakage is not an issue. They likely use stock lenses in many cases, anyway.

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## HarryChiling

> No lab I have ever worked in or run tested every lens surfaced to 1.0 CT. I challenge labs to even be able to prove that ANY testing is done regularly. Smoke- being blown in places unmetionable....


The testing is done once on a number of lenses not just one.




> Cumulative Binomial Sampling Table at 100% Confidence with Zero Consecutive Failures
> 
> # of lenses     -    100% confidence that the % fracture is no worst than
> 
> 120 - 6.5%
> 130 - 6.0%
> 140 - 5.5%
> 150 - 5.0%
> 170 - 4.5%
> ...


 


> (By-the-way: I make 1.56 with a 1.0 CT and it passes drop ball testing at over 100 inches even after A/R.)


 


> I will attest that our testing is documented here for our own tests, and at COLTS for the tests they perform on our products.


I am just saying that the lens you said you made to pass a 100 inch drop ball test could be a fluke.  If you did not perform the test on a number of lenses that would allow you to say with confidence that 99% of the lenses would fracture less than 6.5% of the time in testing.  Then they do not meet 21 CFR 801.410 and are not impact resistant lenses.  The numbers provided above come from COLTS Laboratorys and is their process for testing lenses.  




> Your lab charges for 1.0 CT because they *can*, not because they have to. If they have good processes and controls, breakage is not an issue. They likely use stock lenses in many cases, anyway.


 It is true they charge me for it because they can, that is because it is more difficult to make thinner lenses without running into problems.  I have done surfacing for a number of years and the thinner they get the more headaches I would have to deal with.  This leads to me determining what my machines and staff are capable of without increasing the number of breakages, and if we get a request for thinner you better believe I would charge more for that lens just in case I broke it.  Even the best trained people are going to run into higher breakages when they are trying to surface lenses very thin.  

I believe that you may have made a 1.56 index lens with a 1.0mm CT with AR (assuming both sides wich actually makes the lens more likely to break) and dropped the steel ball from 100 inches on it without it breaking, but if you think I am gullable enough to believe that a lens to this spec can withstand that abuse without a high number of them shattering you are dead wrong.

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## hipoptical

Harry,
If you would read my posts, I have said clearly that we have done extensive testing on a regular basis, not one lens and hoped for the best.
This thread was not supposed to be about what I do, but about how thin one CAN surface different products. The point is, that in the good 'ole USA we have lots of regulations, and other places do not. Those countries that do not have the regulations we have are free to do what they are capable of. I've not heard of an excessive number of injuries to the eye, or of people going blind because they wear thin lenses. The advice the person was looking for was how thin CAN you go... not how thin Harry and the Regulators think you SHOULD go;). 
I push the envelope... sounds like you stay well within the boundaries set. I am comfortable with my position, and you with yours. The difference I think, is that I am  fine with where you are, but you seem to be distraught with  what I say I can do. I would just advise anyone to only work within the boundaries that  one is comfortable with. Some people are  pioneers, some are settlers. I don't settle. 
Unless there's anything more, I think I'm done with this thread, since it seems like the one who started it is gone. I don't want to appear as though you or I are hijacking. (If there is more that needs to be discussed I will be happy to.)

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## hipoptical

> I believe that you may have made a 1.56 index lens with a 1.0mm CT with AR (assuming both sides wich actually makes the lens more likely to break) and dropped the steel ball from 100 inches on it without it breaking, but if you think I am gullable enough to believe that a lens to this spec can withstand that abuse without a high number of them shattering you are dead wrong.


Sorry- but I did mean to respond to this...
Facts:
1.56 index material
A/R both sides (vacuum technology, not nano- or sol-gel)
And it was actually 120 inches, on a number of lenses, quite repeatable, and documented


I don't think you're gullible. I think you accept things that are known. I think you believe that what's normal and widely done is all there is. I think you're a sheep. I think you're a settler.
(And I don't say that harshly or to be rude or demeaning. People are just different. I have to find better and different ways to do things or I get bored. Now, just to make you feel better, you may like to know that we normally still produce this product with CT of 1.8-2.0; just because I can do something doesn't mean I always do. I will do it if asked, though.)
As Red Green says, have fun out there- and keep your stick on the ice.

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## HarryChiling

> Sorry- but I did mean to respond to this...
> Facts:
> 1.56 index material
> A/R both sides (vacuum technology, not nano- or sol-gel)
> And it was actually 120 inches, on a number of lenses, quite repeatable, and documented
> 
> 
> I don't think you're gullible. I think you accept things that are known. I think you believe that what's normal and widely done is all there is. I think you're a sheep. I think you're a settler.
> (And I don't say that harshly or to be rude or demeaning. People are just different. I have to find better and different ways to do things or I get bored. Now, just to make you feel better, you may like to know that we normally still produce this product with CT of 1.8-2.0; just because I can do something doesn't mean I always do. I will do it if asked, though.)
> As Red Green says, have fun out there- and keep your stick on the ice.


 A sheep  :Mad:  , you must be trying to aggrevate me.  Thats OK I am not saying it can't be done, but to have this pass test repeatably, yes I question that.  The fact that you say it passes 100 inches and now its 120 inches.  I have a fishing story that goes the same way every time I tell it :D .  Yes that was a little nip to aggrevate you.  Any way I am sure it ca be done, but you are right I don't think it should be done (opinion).  I have surfaced thinner myself, I once had a lens surfaced so thin that the edges were almost like paper.  I gave the guy the lens didn't charge him, didn't write it up and told him I don't know him.  It's not that I am a settler, I play by the rules.  The fact of the matter is that the US IS obbsesed with impact resistance, and you are absolutely right that it is due to people being sue crazy.  I would rather live within the rules than outside.




> Some people are pioneers, some are settlers.


I would differ in opinion if you are trying to say that surfacing beyond a point is pioneering, it is dangerous to the patient if taken too far.  It is not difficult to surface a lens down that thin, but you are correct in that it would require some knowledge.  Like pressure settings on your polishers, and wich blocks to use to get thin especially on the high pluses.  The lab I worked in before my current position we had the glass coburn blocks and alloy for the pluses so that the diameter of the block was smaller than the wax, but the people that were used to working with the wax would always get waves in their lenses, because they would not let the alloy cool long enough.  These are things that contribute to the fact that if I had to do a high plus it would cost more, the blanks are still the same price, but the process is different and sometimes the settings have to be changed, and everyone in the lab was not familiar with the finess involved with the different processes.  I would have to charge more, because the breakage was higher.  Now I work in a practice that has a finishing lab only and I understand that any lab I order from might charge me a fee or premium for surfacing beyond their limits.




> just because I can do something doesn't mean I always do. I will do it if asked, though


I would hope that if asked, you would charge a fee for this expertise or pioneering that you are doing.  

Like you said in your previous post I think it boils down to opinion.  I guess we learned where each other stands and the guy that started the thread actually didn't get his question answered :cheers: .

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## hipoptical

This last thing....
100 inches, 120 inches? Not a fish story or a change in the truth. Just a flaw in me. If you ask me what time it is, I will say that it is now 1:30, when it is actually 1:21 as I write. The reason I said 120 the last time was to give you the actual facts, not just a rough figure. Our drop ball unit actually goes to 150 inches, but the lenses would not CONSISTENLY pass at that level. That's why I didn't bring that up. I only say this to say that no matter how little you believe, the truth doesn't change. (Many people think Jesus was nothing special, that doesn't affect the truth, either.) Your perception is your opinion, the truth may be different.

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## HarryChiling

> 100 inches, 120 inches? Not a fish story or a change in the truth. Just a flaw in me. If you ask me what time it is, I will say that it is now 1:30, when it is actually 1:21 as I write.


Again that on was a pot shot for the sheep comment :p, no offence meant :D.  I can believe it, but I question the consistency of it passing was all I was saying.

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## hipoptical

I take no offense to any of this discussion; I sense that you are sparring with me in a friendly way, a good discussion with no anger on either side. 
The only problem with a forum like this is that I can't convince you or anyone else that what I'm saying is ALL true. If you were in my facility, I could prove it, but the reality is that it doesn't matter. I am somewhat encouraged knowing that there are some out there who do not believe that I can consistently produce these types of lenses, with consistent results. It just makes it that much better when we do, and other labs are questioned as to why they cannot, but we can...:D

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## harry a saake

Samuel, what you are saying about your patients makes no sense, what do they do walk in to your dispensary with a pair of calipers?, Samuel, how would they know if it was 1.0 or 1.5. The only thing that makes sense that you said was they want the thinnest lenses they can get, but Samuel that also means within reason. The only way i will ever believe what you said is, i want to see that person who can tell .5 mm difference, most opticians cant no less the public.

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## Samuel Jong

> Samuel, what you are saying about your patients makes no sense, what do they do walk in to your dispensary with a pair of calipers?, Samuel, how would they know if it was 1.0 or 1.5. The only thing that makes sense that you said was they want the thinnest lenses they can get, but Samuel that also means within reason. The only way i will ever believe what you said is, i want to see that person who can tell .5 mm difference, most opticians cant no less the public.


You are right that they didn't know if it was 1.0 or 1.5, I just fulfil their request thinner than what they are wearing now. They want it and I can make it, and go ahead by ignoring the safety center thickness, although it doesn't affect much the edge thickness. Once again, I just to satisfy the patients with their new lenses - thinner.

As you know, sometimes the patients bring their new lenses to the other optical shops in order to be confirmed whether they get thinner lens or not.

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