I'm losing it. What's up with PDs and high wrap frames?

[Prentice Pro 9000]

Forgive me, I rarely deal with this and so I've forgotten the rules.

I have a high wrap frame and the PD appears to be very narrow when measured straight on. From what I have seen on this forum (from years back) is that it's supposed to be this way? Maybe?

The problem is that I've not really seen any resources that address this well. And I feel like I'm crazy because I thought I remembered tons of resources on this topic in the past.

Anyways, I looked in systems. Nada. I looked in Optical Formulas. Nada. The old optiboard posts? Not as clear as I'd like

Somebody please set me straight on the topic. Are the PDs supposed to be compensated inwards? Or are they more narrow when you cut them on a high wrap but it's okay in the end?

[DanLiv]

Wrap frames induce BO prism, so BI prism is needed to compensate. It should be surfaced/generated prism for best effect, but of course simply decentering optical centers can achieve that too. If the PDs on that pair are intentionally narrow to produce BI prism they must be plus lenses. If they are nasally decentered minus lenses, that resultant prism is BO, which just exacerbates the wrap prism.

"The rule of thumb is:
For powers <±2.50D, order with 0.25ΔBI in each eye
For powers >± 2.50D, order with 0.50ΔBI in each eye"
https://www.allentownoptical.com/wra...on-sunglasses/

[drk]

THat's a really neat link.

It's a little older, I think, because optician-compensated wrap eyewear is kind of a thing of the past.

You can just order the "branded" stuff like Attitude SV and they do all dat 4U.

[Prentice Pro 9000]

Thank you Dan. to make matters even stickier, this is a progressive. So then it should have BI surfaced into it order to keep the progressive bits in the correct place but also undo the BO prism?

[DanLiv]

Absolutely, in a progressive you can't just fudge the PD, you gotta grind it.

because optician-compensated wrap eyewear is kind of a thing of the past. You can just order the "branded" stuff like Attitude SV and they do all dat 4U.

True, there is little reason to manually compensate anymore. But you gotta know to order that too. I've talked to plenty of opticians who think panto, wrap, vertex and *poof* your eyewear is magically perfect, and they don't even know what Attitude is (or in the old days, Younger Image). -3.00 in an 8 base Varilux Comfort DRx? Garbage, you just destroyed a perfectly good lens by bending it in half and forcing it to do something its not supposed to. Get the designs DESIGNED to be wrapped.

[Prentice Pro 9000]

Absolutely, in a progressive you can't just fudge the PD, you gotta grind it.



True, there is little reason to manually compensate anymore. But you gotta know to order that too. I've talked to plenty of opticians who think panto, wrap, vertex and *poof* your eyewear is magically perfect, and they don't even know what Attitude is (or in the old days, Younger Image). -3.00 in an 8 base Varilux Comfort DRx? Garbage, you just destroyed a perfectly good lens by bending it in half and forcing it to do something its not supposed to. Get the designs DESIGNED to be wrapped.

I'm extremely annoyed as well when people completely forget about the wrapped offerings from companies. I'm even more annoyed when the reps can't explain the difference.

[Jason H]

Not so sure about the compensation from Allentown. Plus lenses have thickness at the OC. The wrap causes more base out prism with plus. There is a wrap calculator in Opticampus if you wanted to play with it.

[Prentice Pro 9000]

Wrap frames induce BO prism, so BI prism is needed to compensate. It should be surfaced/generated prism for best effect, but of course simply decentering optical centers can achieve that too. If the PDs on that pair are intentionally narrow to produce BI prism they must be plus lenses. If they are nasally decentered minus lenses, that resultant prism is BO, which just exacerbates the wrap prism.

"The rule of thumb is:
For powers <±2.50D, order with 0.25ΔBI in each eye
For powers >± 2.50D, order with 0.50ΔBI in each eye"
https://www.allentownoptical.com/wra...on-sunglasses/

so wait…. What about measuring it for final inspection? Should the PD “appear more narrow (even after dotting up with prism compensation)?

[DanLiv]

so wait…. What about measuring it for final inspection? Should the PD “appear more narrow (even after dotting up with prism compensation)?

No you neutralize it at the prism reference point and then any existing prism should be measurable by the displacement on the lensometer reticles. When inspecting always measure where the PDs, seg heights, OCs, etc. *should* be, not where they are.

[DanLiv]

Not so sure about the compensation from Allentown. Plus lenses have thickness at the OC. The wrap causes more base out prism with plus. There is a wrap calculator in Opticampus if you wanted to play with it.

The wrap does cause more base out, that is what they said. To compensate design the lens with counterbalancing base in prism. Opticampus calculators are great, and so is Darryl's Spectacle Optics application, but neither calculates prism compensation for wrap, only the power compensation, which at significant powers is not sufficient for high wrap fits. The 0.25/0.50 rule of thumb works perfectly well though.

[drk]

I think I remember teaching myself that any lens tilt, due to the lens's thickness, requires the BI prism. It is indpendent of lens power. Just a slabby thing. But yeah, 1/2^ is what we always use (used to use).

This is a separate issue from the outsetting of the optical centers. That's just a geometry thing...if you want your patient to get no UNINTENDED prism (which always occurs from wrapping causing decentration [inward] of the o.c.) you have to outset the p.d.s. IIRC, it's usually a mm per eye (based on a typical 15 degree wrap frame).

See the "Oakley's Secret" thread. Should be in the HOF, I believe.

[Prentice Pro 9000]

No you neutralize it at the prism reference point and then any existing prism should be measurable by the displacement on the lensometer reticles. When inspecting always measure where the PDs, seg heights, OCs, etc. *should* be, not where they are.

Soooo in that case, this sunglass that reads 57ish when it should read 62 is definitely NOT correct, right? This is a +2.00 PAL on a high wrap costa.

[Prentice Pro 9000]

The wrap does cause more base out, that is what they said. To compensate design the lens with counterbalancing base in prism. Opticampus calculators are great, and so is Darryl's Spectacle Optics application, but neither calculates prism compensation for wrap, only the power compensation, which at significant powers is not sufficient for high wrap fits. The 0.25/0.50 rule of thumb works perfectly well though.

So I took a +3.50 and -3.50 and tilted them wrap style against a flat plane. I have to say... the mystery only gets thicker; the plus lens cleary produces a BO result, while the minus appears to keep everything still and just adds a pincushion effect to the image.

[drk]

The BO is there with the minus lens, it's just more difficult to notice. Spot up the OC and draw a vertical diameter line through it to the edges and you'll better see it. It's slight, only about 1/4-1/2^, but it's always BO with that tilt angle.

Alternatively, you could quickly flip between positive and negative face form simulation and notice the jump direction left/right/left/right.

[Barry Santini]

First you have to distinguish between the optics of a wrap glass and the mechanics of the same.
The optics pretty well covered above.
For the mechanics, try only thinking about a lens with a non-symmetrical front surface, such as a progressive or fully optimized digital single vision.
When you wrap a frame, you effectively narrow the chord of it’s “PD.”
This must be addressed in blocking by either:
1. Widening the blocking PD to compensate, or
2. Narrowing the DBL, which is functionally the same.

Both of these are mechanical adjustments in blocking purposes only. They would also hold true for bifocals, in theory.

B

[Lensman11]

Barry is correct the issue is how much do you compensate for the wrap. This was an issue before wrap frames became popular when we had to fabricate high base lenses I.e. cataract lenses 12 base and above. The solution we used was to have metal pd rulers curved to different base curves and measure the frame pd with the corresponding ruler to the base cover of the lenses It was even necessary with 8 base curve lenses though the impact was not as great.

[Prentice Pro 9000]

First you have to distinguish between the optics of a wrap glass and the mechanics of the same.
The optics pretty well covered above.
For the mechanics, try only thinking about a lens with a non-symmetrical front surface, such as a progressive or fully optimized digital single vision.
When you wrap a frame, you effectively narrow the chord of it’s “PD.”
This must be addressed in blocking by either:
1. Widening the blocking PD to compensate, or
2. Narrowing the DBL, which is functionally the same.

Both of these are mechanical adjustments in blocking purposes only. They would also hold true for bifocals, in theory.

B

If all is edged correctly with a PAL, then the PD should measure correctly? for instance, take a look at the pic below. It is a PAL and it's supposed to have a PD of 63 put end to end it is measuring about 57

[Robert Martellaro]

If all is edged correctly with a PAL, then the PD should measure correctly? for instance, take a look at the pic below. It is a PAL and it's supposed to have a PD of 63 put end to end it is measuring about 57

The fitting and prism reference point should measure 63 with a tolerance of 0.33 Δ or 1.0 mm, depending on the power. Use a cutout chart to measure.

https://www.essilorpro.com/content/d...91_PRO_VAR.pdf

Best regards,

Robert Martellaro

[drk]

True/false?

On PP9000's job, if the lab simply hasn't compensated for the ~15 degree wrap, the 63 ordered would come back about 61 due to lack of compensation.


True/false?
That huge a deviation from what was ordered exceeds the effect of merely having a wrapped frame. In other words, what could possibly be the excuse for that?

[Robert Martellaro]

I'm not a wrap expert, but to the best of my knowledge the FP and PRP should match the IPD regardless of the degree of tilt.

[drk]

Absolutely right.

The question in the above case is: why didn't that happen? Lab failed to compensate? Lab thought it was already compensated? But under no circumstances should it be off by 6 mm.

[Kwill212]

So I took a +3.50 and -3.50 and tilted them wrap style against a flat plane. I have to say... the mystery only gets thicker; the plus lens cleary produces a BO result, while the minus appears to keep everything still and just adds a pincushion effect to the image.

In Introduction to Ophthalmic Optics (Meister and Sheedy) this is referred to as Prism by Obliquity, Systems for Ophthalmic Dispensing has a section titled "Induced Prism with Wrap-Around Eyewear". This prismatic effect is due to the wrap angle, the front base curve, index of refraction, and the thickness of the lens. The induced prism can be approximated by the equation given by Meister and Sheedy:



Δ = the prism induced
θ = the angle of tilt
t = the thickness of the lens at the reference point
in meters
n = lens refractive index
F1 = the front curve of the lens




As you can see power of the lens for this type of induced prism is irrelevant. The reason there is such a difference between your +3.50 and -3.50 experiment is due solely to the base curve and thickness. The +3.50 is obviously going to have a thicker center and be on a higher base curve. You can see the effect of this type of prism by just picking up any plano lens(high base is better) and tilting it left to right about the 90 degree axis.

As for your other question about the Costa frames. I would not accept that. In my expertise sunglass companies are awful at making and measuring lenses. The way I measure high wrap jobs is to place the 34mm mark of the PD stick on the right lens nasal engraving then measure the the left nasal marking. The influence of wrap between these two points in going to be negligible. You can also just measure the distance between nasal engravings and add 34mm.

*edit fixed equation picture

[Prentice Pro 9000]

Absolutely right.

The question in the above case is: why didn't that happen? Lab failed to compensate? Lab thought it was already compensated? But under no circumstances should it be off by 6 mm.

This is all I wanna know!!!!

[Robert Martellaro]

Human error, resulting in deferred success. Let them go if it happens again.

[Prentice Pro 9000]

As for your other question about the Costa frames. I would not accept that. In my expertise sunglass companies are awful at making and measuring lenses. The way I measure high wrap jobs is to place the 34mm mark of the PD stick on the right lens nasal engraving then measure the the left nasal marking. The influence of wrap between these two points in going to be negligible. You can also just measure the distance between nasal engravings and add 34mm.

Thank you! This company is gaslighting me. It’s some company in Italy that does these glass lenses for Costa. The biggest kicker is that he has a pair of poly costas that came out fine. I even sent them pictures of both dotted up with a PD ruler in front of them and they’re still insisting that these are correct.