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.
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I'm losing it. What's up with PDs and high wrap frames?
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Originally posted by Prentice Pro 9000 View PostIf 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
[ATTACH=CONFIG]15568[/ATTACH]
Best regards,
Robert MartellaroScience is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
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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?
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Originally posted by Prentice Pro 9000 View PostSo 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.
[ATTACH=CONFIG]15566[/ATTACH][ATTACH=CONFIG]15567[/ATTACH]
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 pictureLast edited by Kwill212; 04-26-2024, 12:12 PM.
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Originally posted by Kwill212 View PostAs 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.
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Originally posted by Barry Santini View PostFirst 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.
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
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My first reaction is:
Yes, you have displaced inward the MRP/optic axis on the plus lens by wrapping it. So outset it.
Well, the effect on the minus lens is less so. Maybe outsetting is not as important on a minus lens.
I would assume that it's calculated mathematically. Maybe a rule of thumb is not a great approach.
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