# Optical Forums > Progressive Lens Discussion Forum >  Tolerance  for optical center placement (PD) on progressive segs

## rdcoach5

I was always taught 1/2 mm tolerance on PD but that was in the day of Varilux Infinity . What is tolerance now?

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## Judy Canty

With the acquisition of Frames Direct and their "algorighim", PD and seg placement may become a moving target.

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## rdcoach5

> With the acquisition of Frames Direct and their "algorighim", PD and seg placement may become a moving target.


Seriously, what do you lab guys consider tolerance for horizontal PD placement? Darryl, do you have an opinion?  My local lab tried to tell me 2mm per eye and I thought he was joking. Why do I try to be accurate to 1/2 mm when the lab can then be off 2mm either way? I know the pt can get used to it that way and I also know it limits the width of the int especially if it is off. So what is the official tolerance?
       Thanks, Bob

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## Leo Hadley Jr

> With the acquisition of Frames Direct and their "algorighim", PD and seg placement may become a moving target.


 
:cheers: lol

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## Speed

We always went with 2 mm total.  I knew one tech rep that recomended taking the near and adding 2.5 mm to each eye calculate the distance.  He felt the near placement was most important.  I am sure this did not work in all cases.

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## WFruit

*Cue Family Feud Music

Survey says: Anzsi Z80.1-2005 

 0.00 to +/-3.75 Tolerance = .67^  
 over +/- 3.75 Tolerance = 1.0mm from specified monocular interpupillary distance

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## Mr. Finney

The sad part is, I was actually humming it while I was reading!  But on a progressive, I think the number should be 1mm per eye.  You can't be off 2/3rds of a prism diopter on a progressive and still have a functioning lens, IMO.

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## Uilleann

I think the best labs are the ones who don't work in "tolerances" but rather they simply try to get it perfect and exact.  Every single time.  Period.

They won't of course, being as they're human, but if they're trying, you will have far fewer problems down the pike.  That said, with *ANY* lens, I would start to raise an eyebrow if we were consistently seeing work back that was anything greater than 1mm off in a given lens, in any direction: PD, OC, SEG etc.  Just another dispenser's 2 pence.

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## WFruit

> I think the best labs are the ones who don't work in "tolerances" but rather they simply try to get it perfect and exact. Every single time. Period.
> 
> They won't of course, being as they're human, but if they're trying, you will have far fewer problems down the pike. That said, with *ANY* lens, I would start to raise an eyebrow if we were consistently seeing work back that was anything greater than 1mm off in a given lens, in any direction: PD, OC, SEG etc. Just another dispenser's 2 pence.


Well, yes. "You are correct sir!" (going back to the old SNL spoofs of the Johnny Carson show....)

But the question was about what the Tolerance _IS_, not what it _SHOULD_ be.:p

Personally I agree that 1mm is best, and it's what I've always used when I do Final Inspection here (not my actual job currently, but since I _can_ do it, I get stuck out there when they're short handed (or short eyeballed....)).

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## Uilleann

There are labs out there that can and in fact DO get it right almost 100% of the time.  If you're getting less than stellar results from your own supplier, a change may well be in order.  If YOU happen to be doing the layout and edging, then look to improve your own skills in any way you can, perhaps look to more accurate equipment for the types of lenses you're running (expensive, but if its a big problem, worth the investment), and take enough time and care to see that each job is done as perfectly as you can manage.  Raise that bar!  :)

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## Uncle Fester

Tolerance is often subjective.

My picky patients need 1/2mm or less.

My easy patients tolerate 2mm or more.

My impossible need a week on the shelf after they complain and then "These are much better"!

Who's who with a new patient? There's the rub.:)

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## YrahG

> *Cue Family Feud Music
> 
> Survey says: Anzsi Z80.1-2005 
> 
> 0.00 to +/-3.75 Tolerance = .67^ 
> over +/- 3.75 Tolerance = 1.0mm from specified monocular interpupillary distance


That's for a progressive the SV an MF is:




I personally believe the 2.5mm is too much, especially when the PAL can be made to within a 2.0mm tolerance consistently.

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## Darryl Meister

Just keep in mind that there is now a tighter tolerance on the centration of the prism reference point (1 mm per eye instead of 2.5 mm combined) with progressive lenses in the ANSI Z80.1 standard but a slightly loser tolerance on the measured prism imbalance in higher powers in order to keep verification relatively straightforward.

The unwanted prism in progressive lenses is essentially measured in the same way that you would measure prism in an uncut, at the prism reference point of each lens. The prism tolerances are applied to these measurements, and separate centration tolerances are applied to the location of each prism reference point. This ensures tighter centration accuracy, which is especially critical with progressive lenses.

A summary of the specific tolerances can be found at: *OptiCampus.com ANSI Z80.1 Summary*.

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## Barry Santini

I feel we need to keep separate the tolerance described here, used in _fabrication_, from that used when obtaining the _measured_ PD.  By this I mean that a 1mm tolernace in fabrication is totally fine, as long as the value you supply is representative (some people say accurate).

But, even today, how do we know what is really ideal, when vertex and final lens bevel mounting are not normally factored in?
Further, miscalibrated pupilometers, un-square forehead anatomies and nasal *turns*  (the reference points for most pupilometers) can all sum up to help deviate our obtained measurements from the theoretical ideal for an individual client.

Good digital centration devices can surmount these traditional obstacles to obtaining the best PD values.  As for heights, I too believe that digital centration can be really excellent here.  But...anyone who uses a raw height in _isolation_, that is, without factoring in postural habit, eyewear fit and previous placement invites potential client dissatisfaction.

An unfavorable mismatch of measurement and fabrication is what *I* think is most responsible for client fussiness.  Certainly, the 1/2mm fussiness described above could stand some serious investigation.

B

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## Barry Santini

Darryl, while your eyes are on this thread, I've been thinking....

Wouldn't a spherical-trigonometric-derived PD adjustment for blocking wrap eyewear deliver better values than the simple trig formula that's been recommended up to now?

Barry

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## Darryl Meister

You may need to clarify what you mean by "spherical" in this context. You could be referring to the effect sof tilting a curved lens or to the fact that wrap tilt swings the optical center back along an arc or to something else entirely...

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## Barry Santini

I was referring to "that wrap tilt swings the optical center back along an arc "

Thanks in advance

B

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## musicvirtuoso

> I think the best labs are the ones who don't work in "tolerances" but rather they simply try to get it perfect and exact.  Every single time.  Period.
> 
> They won't of course, being as they're human, but if they're trying, you will have far fewer problems down the pike.  That said, with *ANY* lens, I would start to raise an eyebrow if we were consistently seeing work back that was anything greater than 1mm off in a given lens, in any direction: PD, OC, SEG etc.  Just another dispenser's 2 pence.


ANSI Tolerances notwithstanding, I agree whole-heartedly with Uilleann. My theory is that we shouldn't be fitting, surfacing, or edging to tolerance, but rather to what is prescribed. I understand that sometimes, getting things perfect is just too difficult and costly - and that's where tolerances do come in. I personally strive for perfection and if that is not met, I measure the effect on that particular patient. If the effect is significant, or even noticeable, I consider a redo. This may not be possible for everyone to do, but I still believe it's what we should be striving to do. It's so hard to put a number on this kind of thing. What 2mm does to one person may be multiplied 10 times in another. So personally, I really hate referring to standardized tolerances. And that's my two cents...

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## OpticalMessiah

Tolerances are indeed good guidelines, but the patient's quirks also need to be considered. For many hyperopic patients, I used the near PD +2.5mm/eye for years and it worked most of the time. As lenses become more sophisticated, I think we also must be more "picky" (for want of a better word) with these measurements. Matching the PD and PRP from the patient's past glasses is a good start, but it doesn't always guarantee success. A new patient with PALs is the most difficult, particularly if the patient is an absolute presbyope, of course. (I'm also referring to patients who switch lens brands.) My own experience has been that with a myope, you really have to nail it at distance PD. With hyperopes, nailing the near and letting the particular decentration of the lens create the distance. Then, you've got to know what lenses provide the decentration that you require, so you don't create unwanted prism at distance. As I recall, a lot of these patient idiosyncracies were to be taken care of with the Varilux Ipseo lens, and the measurements were taken care of.
I really had no success with hyperopes with that lens, which I still think is a bit odd. That's my one cent!...

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## musicvirtuoso

> ... As I recall, a lot of these patient idiosyncracies were to be taken care of with the Varilux Ipseo lens, and the measurements were taken care of.
> I really had no success with hyperopes with that lens, which I still think is a bit odd. That's my one cent!...


Very interesting that you put that... I recently posted a question about problems with hyperopes in the Zeiss Individual in the PAL category of the forum ... maybe the problem is similar?

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## OpticalMessiah

I don't think I can respond to the Ipseo v. Individual conundrum, inasmuch as I only fit a handful of Gradal Tops and GT2s with no patient complaints. As with all lenses, I tried these lenses on both myopes and hyperopes, with a few established patients thrown as a control.
My problem with "individualized" lenses is this: the lens manufacturer plays a game of "musical base curves" or "pin the tail on the power", which is not always in the patient's best interest. If you had any experience with the Ipseo, you know that it came from the lab with different powers than originally prescribed to meet the patient's individual needs, supposedly, and you verified the lenses using a decimal scale, instead of quarters and eighths. Sometimes the lenses had "mystery prism", once again to match the patient's head movement. Often, this gave the hyperope a tiny reading area (even with a +1.75 Add) that was quite disconcerting. (Full disclosure: I didn't know what was happening there until I reviewed prism thinning, and the dos and don'ts.) Needless to say, I let the Ipseo go after giving it a good road test. It was OK for myopes, but not hyperopes, generally.
Does the Individual have the same sort of characteristics? Does it work for myopes?

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## musicvirtuoso

Since this is straying from the subject of this thread (and I still want to discuss it), I've created a new thread here: http://www.optiboard.com/forums/thre...912#post343912

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