# Thread: Near PD/ Distance PD relationship

1. ## Near PD/ Distance PD relationship

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

Carry on.

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

3. Originally Posted by For-Life
I would never use a formula for a near pd. I find that they tend to be different

:cheers:

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

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

or this:

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.

5. 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.

6. 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.

7. Originally Posted by drk
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.

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

9. 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)

10. 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?

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

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