Near PD/ Distance PD relationship

[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??

[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.

[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

[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?

[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.

[Barry Santini]

Darryl,

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

Barry

[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.

[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.)

[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.

[drk]

:)That's why they turn their head.

[HarryChiling]

:)That's why they turn their head.

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

[drk]

Academic. No such thing, anyway.