Help with PD

[eyemaniac]

What is the correct method of measuring a PD on a 8base wrap? Do you bend the PD stick around lens or do you measure straight across and eyeball it?

[jamie w]

why would you be taking a PD with the frame on the pts face?

[Leo Hadley Jr]

I think they are trying to measure the PD on a finished pair of glasses.
Go ahead and bend it, the ruler does not get longer when its bent.;)

[scriptfiller]

Are you needing the correct measurement to decenter the lense?

If so, I use the Left nasal eyewire as a balance point (fulcrum). I then align the 0 (zero) at the inner nasal eyewire of the right eye and eyeball the measurement at the inner eyewire of the left eye. Seems to work, rarely do I ever have a re-make fro PD.

[eyemaniac]

Thank you, I thought the same thing

[rbaker]

The difference will be insignificant. Measure it either way.

[DragonLensmanWV]

I think they are trying to measure the PD on a finished pair of glasses.
Go ahead and bend it, the ruler does not get longer when its bent.;)

But it does create an arc with the start and end points closer together than the length of the arc.

Take a flexible PD stick and bend the ends around until they touch. Technically you have a 160 PD if you follow the length of the rule, even though the end points are adjacent.
So I say don't bend the stick.:cheers:

[eyemaniac]

So if you dont bend the PD stick your kind of guessing at the PD.

[scriptfiller]

If measuring to verify PD, I don't "bend the stick". It's kind hard to do so, all mine are metal.

[eyemaniac]

The plastic ones from hilco work great

[finefocus]

[QUOTE=eyemaniac;312592]So if you dont bend the PD stick your kind of guessing at the PD.[/QUOTE
Imagine a stick laid across the back side, from eyewire to eyewire (if the frame were a bow, this would be the bowstring). With your left eye, lay zero on the ruler opposite the right OC dot while sighting perpendicular to the bowstring. Use the right eye to again look perpendicular to the bowstring, through the left dot, and read the ruler. You've just eliminated the parallax inherent to measuring a curved surface with a straightedged ruler.

[Striderswife]

I had been thinking of asking a similar question, but about the "A" measurement of a wrap frame (or even an opthalmic with a little bit of curve). In most frames, the "A" is around 2mm shorter than the actual printed eyesize. We transmit our jobs through Vision Web, and it asks for the "A." So for a slightly wrapped frame, do I bend the ruler around, or just visually eyeball the markings on the ruler and call it as I see it?

To answer the original question, I'd keep the ruler straight for measuring the PD. If our eyes were around our head, like a deer, then the PD would be taken around, right? I just made myself giggle, thinking about if we all had heads like deer. :D

[eyemaniac]

I never thought I would get so many different answers to this question, seems as though nobody all agrees with the same method. Very difficult to know what to do.

[Uncle Fester]

I never thought I would get so many different answers to this question, seems as though nobody all agrees with the same method. Very difficult to know what to do.

Opticians not in uniform agreement?:drop:

:cheers:

[Diane]

I hope I'm understanding that you want the patient's PD. If not my post won't be accurate. Wrap will affect how the patient sees. If you wrap the ruler, then you will get the wrong measurement. Not only will the PD be affected, but also the actual power. Power compensation will need to be done.

Here are some rules of thumb...actually taken from the Advanced Opticians Tutorial.

Ø The optical center should be 1mm lower than the pupil for every 2º of pantoscopic tilt.

Ø The optical center should be 1mm higher than the pupil for 2º degrees of retroscopic tilt.

Ø The optical center should be in 1mm from the pupil for every 2º degrees of positive face form.

Ø The optical center should be out 1mm from the pupil for every 2º degrees of negative face form.

Further, when teaching to measure a frame PD, I instruct to measure from the back side rather than the front and NEVER wrap the ruler.

Diane

[eyemaniac]

This is a PD on a finished pair of glasses

[DragonLensmanWV]

[QUOTE=finefocus;312597]

So if you dont bend the PD stick your kind of guessing at the PD.[/QUOTE
Imagine a stick laid across the back side, from eyewire to eyewire (if the frame were a bow, this would be the bowstring). With your left eye, lay zero on the ruler opposite the right OC dot while sighting perpendicular to the bowstring. Use the right eye to again look perpendicular to the bowstring, through the left dot, and read the ruler. You've just eliminated the parallax inherent to measuring a curved surface with a straightedged ruler.

Good analogy. Now to further this analogy, imagine the pd stick as the bow and the actual pd as the bowstring. They're not the same length are they? The bow is longer than the bowstring, so if you use the pd stick like the bow instead of the bowstring, you will get an enlarged measurement that is not accurate.:cheers:

[finefocus]

**

[chip anderson]

We do negative face form? Retro-tilt is a bit more than I can take. Negative face form I have only seen on glasses that have been in the sun for many years and a few cheaply made frames that needed fixin.
I guess even us old codgers can learn every day.
Anyone want to tell me an sort of a reason good or bad for negative face form?

Chip

[eyemaniac]

The census seems to be not to bend the PD ruler.

[Diane]

We do negative face form? Retro-tilt is a bit more than I can take. Negative face form I have only seen on glasses that have been in the sun for many years and a few cheaply made frames that needed fixin.
I guess even us old codgers can learn every day.
Anyone want to tell me an sort of a reason good or bad for negative face form?

Chip

Chip,

I can't think of ANY good reason for negative face form. It's only included to be all inclusive...for whatever reason...;)

Diane

[eyemaniac]

How to verify PD in glasses all done

[finefocus]

Chip,

I can't think of ANY good reason for negative face form. It's only included to be all inclusive...for whatever reason...;)

Diane

Very extreme poofy-cheek syndrome, combined with depressed nasal structure (often has narrow PD associated). The alternative is huge vertex shift with pad arms pointed straight back. Call it the Pacific Rim Fit.

[DragonLensmanWV]

Very extreme poofy-cheek syndrome, combined with depressed nasal structure (often has narrow PD associated). The alternative is huge vertex shift with pad arms pointed straight back. Call it the Pacific Rim Fit.

Pssst! Harry uses a small amount of negative face-form in his glasses for a reason.

[braheem24]

This is a PD on a finished pair of glasses

Dot the lenses, Measure it from the backside and call it a day.