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

[Kwill212]

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

I'd love to get some takes on the highlighted comment. Obviously increasing the wrap angle will reduce the frame PD as measured in reference to the 0° wrap line. In other words the temporal edges of the lenses get closer together as you described above. However, depending on the base curve, and seemingly the back curve of the lens, the MRP may be displaced inward OR outward when inducing wrap. I have attached a few diagrams to show what I mean. There are 2 lenses, both 8 base, one plus one minus, both with the same A measurement. Both lenses in the diagrams are right lenses from a top down view. The first diagram shows what they look with zero degrees of wrap. The second set shows what they look like with 15 degrees of wrap. The last two are just close up of the MRP displacement.

As you can see both have shortened the chord length of the frame pd by ~2.1mm. But, the minus lens MRP has been displaced toward the temporal side by about 0.6mm and the plus lens has been displaced toward the nasal side by about 2.0mm. The MRP of the minus lens travels in an arc first displacing it temporally then as the wrap increases it eventually travels back nasally until it reaches it original decentration. In this case about 27 degrees of wrap would be needed to return to its original decentration. Increasing the wrap further with then start to decenter the MRP nasal of its original position.

For the plus lens with the plano back curve, the MRP only displaces nasally, because the back curve of the lens has already started at the zero degree wrap line. I've used plus and minus here but it seems that any lens with a minus back curve would initially move the MRP temporally. The steeper the back curve the more magnitude of displacement temporally, and the more wrap required to displace the MRP nasally. Someone tell me I'm nuts and please show me why.

[Barry Santini]

Methinks the reference for these calcs should be the anterior/front surface and not the back surface

[drk]

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.

[Robert Martellaro]

Cosine of tilt angle seems to work.

I don't think ± matters as long as the lens shape is meniscus.