# Optical Forums > Ophthalmic Optics >  Near PD/ Distance PD relationship

## drk

If I did this right, the near p.d. will be 95% of the distance p.d. for 40 cm. reading distance.

Carry on.

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## For-Life

I would never use a formula for a near pd.  I find that they tend to be different

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## Jacqui

> I would never use a formula for a near pd.  I find that they tend to be different



I agree, measure instead. 

:cheers:

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## HarryChiling

> If I did this right, the near p.d. will be 95% of the distance p.d. for 40 cm. reading distance.
> 
> Carry on.


No, especially when you factor in prism through various powers.  Check this out:

http://onlineopticianry.com/wordpres...?s=inset+table

or this:

http://www.optiboard.com/forums/showthread.php?t=30720

The table that I made for optiboard also shows you when the lens is outside of the tolerance when using the most common cheat of 1.5mm inset for reading.  It also shows the powers that arethe most accurate when usign these powers, of course the viewing distance will affect the outcome so you can modify that as well and see what's up.

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## fjpod

I always measure, but I have always found...

little person NPD is 3 mm smaller
big person NPD is up to 5 mm smaller

There are exceptions with tropes and CIs, and such, but in those cases you're probably going to have a hard time deciding what PD to measure exactly anyway.

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## drk

> _You can also measure a Distance PD and calculate the Near PD for any reading distance using:_
> 
> _Near PD = [Distance PD mm * (10 * Reading Distance cm)] / [(10 * Reading Distance cm) + 27]_
> 
> _where the Reading Distance is given in centimeters and the PDs are in millimeters._


Is this incorrect? I pulled it off this board...


I think what you're referring to, Harry, is how to compensate a near pd in layout to make it match what is specified. (?)

What I'm doing (I think) is finding a way to estimate near pd with accuracy given only the distance p.d., a situation which occurs in optical from time to time.

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## HarryChiling

> Is this incorrect? I pulled it off this board...
> 
> 
> I think what you're referring to, Harry, is how to compensate a near pd in layout to make it match what is specified. (?)
> 
> What I'm doing (I think) is finding a way to estimate near pd with accuracy given only the distance p.d., a situation which occurs in optical from time to time.


Yup, that's right the 27mm isthe center of rotation to back vertex of the lens so depending on what book you pick up you will see that figure anywhere from 27-30mm.

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## drk

If you agree, use 40 cm for reading distance solve the equation npd/dpd.  What do you get?

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## HarryChiling

Near PD = [Distance PD mm * (10 * Reading Distance cm)] / [(10 * Reading Distance cm) + 27]

Dpd of 55mm (I would say the lower end of a normal PD range)

Npd = 27.5 * 400 / 427
Npd = 25.76

25.76/27.5 = 94%

Dpd of 72mm (I would say the top end a normal PD range)

Npd = 36 * 400 / 427
Npd = 33.72

33.72/36 = 94%

Well what do you know Drk you may be on to something. Actually suprisingly I don't know why I didn't see this before the following ratio holds true for any NPD/DPD

Npd/Dpd = Working Distance in mm / (Working Distance in mm + Vertex to Center of Rotation)

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## drk

You may understand the dynamics better than I, but we have a modest level of accuracy needed.

We always, for better or worse, take near pds at 40 cm, and that's a real source of error. But who is going to average six working distance measurements of the patient holding a Ladies Home Journal, anyway?

What is the effective power change of a 2 mm nosepad slip in a -8.00/+2.00 add?

Ad nauseum.


Sure, measure near pd, if you must. I still will. But realize in most cases near pd is only used for segmented MFs, anyway, and for my part, I always use binocular measurements so the segs look symmetrical, prism be darned. 

Another simplification of complex operations.

Just don't tell the folks at GlassesSellersOnline. They'll make a big deal about their new secret formula  :Rolleyes:

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## Darryl Meister

> Well what do you know Drk you may be on to something. Actually suprisingly I don't know why I didn't see this before the following ratio holds true for any NPD/DPD


That's a very nice rule of thumb from you two. Geometrically speaking, 94% represents the ratio of the triangle formed between the near fixation point and the two centers of rotation to the similar triangle formed between the near fixation point and the spectacle plane. Of course, above a +2.50 D addition, which represents a limit on the depth of field of 40 cm, the working distance (in meters) becomes equal to the reciprocal of the addition power.

But keep in mind that this relationship still does not consider the effects of prism induced by the distance prescription on near convergence. The exact formula for the near PD is given by:



where W is the working distance in millimeters, s is the stop distance (27 mm) in millimeters, and F is the power of the lens.

As a simple rule of thumb, however, I would usually just add 0.5 mm to each inset for plus prescriptions and subtract 0.5 mm from each inset for minus prescriptions.

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## drk

Yowza.

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## HarryChiling

Darryl, that's what I used for my inset table. I found that many opticians in the various offices I worked used a rule of thumb of 1.5mm inset per eye for the near PD which coincidentaly is roughly 94% of an average PD, but as the power of a lens goes up the prism cannot be ignored so the table was to demonstrate and to show when the lens would be out of tolerance using ANSI, given your rule of thumb of adding or subtracting 0.5mm I believe that would cover the full average prescription range and still keep it under the threshold of ANSI.

Can't take any of the credit that was all Drk.

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## MarcE

go slow for me. 

So in order to compensate for some of the prism, we should subtract an extra 0.5mm each eye for myopes and add the same for hyperopes??

But that doesn't have anything to do with the relationship with DPD and convergance at 40cm, which we have discovered is 94%.

Does the surfacing lab normally take this into account based on the DPD and the power?  And if they do, it is relayed to blocker?  It isn't relayed to this blocker.  Mine blocker is set at the rule of thumb of 1.5mm each.

Explain. Thanks

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## HarryChiling

> go slow for me. 
> 
> So in order to compensate for some of the prism, we should subtract an extra 0.5mm each eye for myopes and add the same for hyperopes??
> 
> But that doesn't have anything to do with the relationship with DPD and convergance at 40cm, which we have discovered is 94%.
> 
> Explain. Thanks


Your right it has nothing to do with the convergence untill the Rx lenses are in place, then they have a prismatic effect that the eye has to compensate for by converging more or less, the lenses can be decentered more or less so that the eye's don't have to do the compensateing.

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## MarcE

Now let's talk practicalities.
For blocking should I reduce the DPD by 0.5mm each for hyperopes and increase it by 0.5mm for myopes?  Even though I use the DPD to enter into the layout blocker, as you know the seg is used for the layout.

If the compensated NPD is entered during surfacing, and then it is blocked according to the 1.5mm rule of thumb, we haven't helped anything except now both the NPD AND the DPD are off by 0.5mm??

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## Darryl Meister

> For blocking should I reduce the DPD by 0.5mm each for hyperopes and increase it by 0.5mm for myopes? Even though I use the DPD to enter into the layout blocker, as you know the seg is used for the layout. If the compensated NPD is entered during surfacing, and then it is blocked according to the 1.5mm rule of thumb, we haven't helped anything except now both the NPD AND the DPD are off by 0.5mm??


If you increase the distance PD while blocking for surfacing, you would need to decrease the near PD while blocking for finishing by an equal amount, and vice versa if you decrease the distance PD while blocking for surfacing. In either case, the segment inset is increased (or, conversely, decreased) accordingly during both blocking stages, which will maintain the correct distance PD and the properly compensated near PD.

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## Barry Santini

The issue I have with all the *calculator* approaches is the assumption that this level of *precision* is a reasonable goal.

In the end, what we really need is a completely *subjective* way of determining PDs for a particular client, Rx, VD, and near point.

The measurements that Designs for Vision uses for their Bioptics would be a good start, IMHO, if what you're seeking is the ultimate in (corrected from my original post) *representative and correlative* measurement accuracy.

Barry

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## MarcE

> The issue I have with all the *calculator* approaches is the assumption that this level of *precision* is a reasonable goal.
> 
> In the end, what we really need is a completely *subjective* way of determining PDs for a particular client, Rx, VD, and near point.
> 
> The measurements that Designs for Vision uses for their Bioptics would be a good start, IMHO, if what you're seeking is the ultimate in accuracy.
> 
> Barry


Exactly what I was thinking after talking with one of my lab guys.  The Far minus 3mm relationship works fine for 85% of Rxs.  And another 14.5% of the Rxs are able to compensate for any small amount of induced prism.  The trick is to recognize the rare Rx/patient combination that will cause problems.  We have found a few of these.  Normally the way we find them is that they have been to other Drs/opticals and they were never happy even after re-refractions and remakes.  Our Dr finds no significant difference in the Rx so we start with stuff like the lens design, induced prism, BCs, and Near PDs.

This thread reminds me of a patient that had similiar symptoms.  Progressive wearer that had trouble at near and especially intermediate.  High cyl.  He had a wide PD of 72.  We changed him out of poly, we checked the PD and it was spot on time after time.  We changed lens designs and BC.  These things helped somewhat, but not completely.  Of course he was converging more than 3mm at near.  It makes sense now.

Do these new fancy custom progressives take this into account for wide or narrow PDs?

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## Darryl Meister

> The issue I have with all the *calculator* approaches is the assumption that this level of *precision* is a reasonable goal.


Just keep in mind that a more accurate estimate for the near PD initially will result in a final product that comes closer to the correct PDs. Besides, there is always the potential for centration errors during the finishing process, which can serve to exacerbate any differences between the calculated near PD and the correct near PD.

The biggest advantage to an accurate _subjective_ near PD measurement, in my opinion at least, would be the potential to compensate for any habitual head turn or fixation disparity.




> Do these new fancy custom progressives take this into account for wide or narrow PDs?


Some do, some don't. You should check with your free-form lens supplier of choice.

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## Barry Santini

Darryl,

I agree with your commnts 100%!  I think *I* am just math-phobic!

Barry

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## Andrew Weiss

OK, a real-world example and question:

Here's the patient's Rx:

-9.75 -2.25 X019
-13.75 -1.25 X002
Add 2.00

PDs, measured monocularly using reflection pupilometer and near fixation at 40mm:

Distance: 28/29
Near: 26.5/28.5

Patient complaint was that she had difficulty finding the reading area on her previous progressives, which had a similar Rx.  The lens used was a Panamic 1.74, and the distance PD was used, which that optician measured at 29 and 30.

Because her Rx leaves out the free-form generated lenses, I elected to change her to a Physio 1.74.  To compensate for the unequal convergence, I elected to use PDs of 27.5 and 29.5, knowing that they might be a bit off on the distance but that the seg inset would be more in line with how her eyes work.

Now I'm wondering, among other things, whether I ought to have added in prism because of the Rx.

Comments and suggestions, please?  Thanks, all.

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## drk

I'm not sure I buy into the "unequal convergence" concept.  I think it's error in measurement.

Even if it were present, it would, as Darryl said, result in a head turn towards the side that converges less.  Who does that?  I've not seen it.

If the theoretical unequally-converging patient's progressives were made with symmetric inset (i.e., the regular way), the worst that would happen would be elimination of the habitual head turn.

How many times have you seen a patient at dispense say "I have to turn my head to see clearly" and they demonstrate by _straightening their head?_

(I've seen the opposite, though...having to make a head turn off-centerline to see clearly.  That one bugs me alot.)

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## HarryChiling

> Even if it were present, it would, as Darryl said, result in a head turn towards the side that converges less. Who does that? I've not seen it.


Or reduced field of vision in the near gaze.  Same issue that occurs with FT or TR lenses when the insets are fudged to make them more cosmetically appealing it reduces the binocular field of vision.

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## drk

:)That's why they turn their head.

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## HarryChiling

> :)That's why they turn their head.


Which only corrects for one eye when using symetrical seg insets.  In essence monovision progressives.

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## drk

Academic.  No such thing, anyway.

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