PALs Bi-Convex at Near?

[AndyOptom]

Maybe I'm overthinking this one, but here goes!

So I was having a pleasant catch-up with a gentleman who works for Hoya, and we ended up discussing that golden oldie, the Hoya ID. Specifically, how the add is achieved by plus-cyl at right angles to each other between both surfaces (in effect, a customized cross cylinder), so the front near portion is convex in one meridian, and rbe back is convex along the perpendicular meridian.

So far so good, that part wasn't a problem for us.

Then we started wondering... for full back surface PALs, the front base curve must be sufficiently convex so the back remains concave. Every case where a flatter blank has been used on such designs (leading to a biconvex near portion), has led to complaints and an eventual rejection of the lens in favour of a move convex front with the concave back. This is logical due to ocular rotation etc, of course.

Is it as simple as combined plus cylinders between both surfaces behaving very differently than a lens that is biconvex in all meridians?

Granted, we both have dispensed many pairs of ID, the patients generally love them as much as we don't let our fitting slip up... so it's more an academic curiosity if nothing else.

[Lensman11]

Every myopic rx that is a full back side progressive will be convex on the inside to create the needed plus power. It does not seem to cause a problem probably because the field of view is so small

[Kwill212]

Every myopic rx that is a full back side progressive will be convex on the inside to create the needed plus power.

Surely you mean it will be less concave in the near portion, not convex. Unless the base curve is less than the total read power, which would be weird.

-6.00 sph +2.50add on a 2 base. Where is the convex back portion? Would it not just go from a -8 back curve in the distance to a -5.50 back curve in the near portion?

[optical24/7]

Well…..It depends on lens/powers and base curve. In the pursuit of thinner lenses, designers have been reducing the plate height by dropping BC’s from best curve and compensating with FF produced aspheric/atoric designs. With minus Rx’s your still gonna be “less concave” along the umbilic. But take an Rx of +2.50 with a +2.50 add FF’ed on a 4 base.

If you use simple 1.53 index calc’s, you have to put a +1.00 in the umbilic of that particular script to create the +5.00 you need at near.

[Kwill212]

Well…..It depends on lens/powers and base curve. In the pursuit of thinner lenses, designers have been reducing the plate height by dropping BC’s from best curve and compensating with FF produced aspheric/atoric designs. With minus Rx’s your still gonna be “less concave” along the umbilic. But take an Rx of +2.50 with a +2.50 add FF’ed on a 4 base.

If you use simple 1.53 index calc’s, you have to put a +1.00 in the umbilic of that particular script to create the +5.00 you need at near.

Well...... Yeah. That's why I was responding to the "every myopic Rx" part. Which makes no sense.

[optical24/7]

Well...... Yeah. That's why I was responding to the "every myopic Rx" part. Which makes no sense.

I got what you said, (correctly), I was just adding context to the OP’s quarry for future readers…It is “possible” to have a bi-convex designs with total FF concave side processing.

[Robert Martellaro]

Andy,

WRT Hoya ID-

In the case of the Hoyalux iD Integrated Double Surface progressive design5 shown in figure 7(b), the full reading addition is obtained by incorporating the vertical component of the addition on the convex surface and the horizontal component on the concave surface. It can be seen in figure 7(b) that the convex surface increases in power from +5.00 D to +7.00 D along the vertical meridian, but there is no change in power between distance and near along the horizontal meridians in the intermediate and near zones, which remains +5.00 D. Since the back surface is also -5.00 D along the vertical and horizontal meridians in the distance zone, this area of the lens has zero power.

https://www.opticianonline.net/cpd-archive/187

Going convex in the near zone on the back surface is certainly a no-no.

However, rear surfaces with reversal of curvature exhibit specific wearing properties which can lead to problems for the spectacle wearer.

https://www.optiboard.com/forums/showthread.php/66146-Camber-Lens-Seeking-Feedback?p=536029&viewfull=1#post536029

Best regards,

Robert Martellaro

[drk]

Can we dumb it way down for me?

From what I'm hearing:
1. Having a bubble on the backside of a PAL lens is bad (like an old Ultex BF)
2. It could happen when the lens is fairly plus in the distance with a higher plus add. Lots of plus.
3. Most spherical front surface FF PALs just have to cope.
4. But putting some plus back on the front side (which everyone used to do but tries to avoid now because it's cool) eliminates the bubble.
5. Hey, Camber has some front side plus progression and that's why it was awesome (but it's not popular, now*)

Is this the gist?
If so, more questions for you all.

*Right?

[Lensman11]

Here is my spin on the issue. It does not happen that often only when the total reading power is greater than the front base curve. It is does not seem to effect the wearability of the lenses as we are talking about the a very small field of view so ignore the problem if the patient is complaining it is most likely not this issue or raise the base curve and eliminate the issue.

[ChrisBowers]

Labs try to choose a front curve that necessitates an inside base curve of around -6.00. Any add only reduces the integer by that plus amount. Using a -1.00 inside curve with a ff pal … well you just broke the corrective lens theory. So, for the 2% of the population where the Rx conflicts this, I’d fit contacts. I will say in recent years labs have lowered front curve by about 1 diopter to accommodate flatter frame curves.

Chris

[Robert Martellaro]

Can we dumb it way down for me?

From what I'm hearing:
1. Having a bubble on the backside of a PAL lens is bad (like an old Ultex BF)

Yes, the vision gets a tad wonky using an s-shaped curve on the back, with increased risk of "lash crash".

2. It could happen when the lens is fairly plus in the distance with a higher plus add. Lots of plus.

Even low plus forces a steeper base curve than FS PALS.

3. Most spherical front surface FF PALs just have to cope.

And does so by bumping the base curve, the refractive index (with increased chroma and expense), or both.

4. But putting some plus back on the front side (which everyone used to do but tries to avoid now because it's cool) eliminates the bubble.

Yes. More wiggle room so to speak.

5. Hey, Camber has some front side plus progression and that's why it was awesome (but it's not popular, now*)

Maybe because of the licensing fees? Lack of technical knowledge out here in the field?

Best regards,

Robert Martellaro

[drk]

You guys are great, but I have to mansplain it to myself.

To synthesize your posts:

1. The preferred limit to the inside curve of a FF PAL is 6D concave.

2. That means you may manipulate the front/base curve by steepening for in some plus Rx cases.

3. For example (math warning) assuming a plano with +2.50 add, the front curve would have to be ~+8.50 to meet the "no flatter than -6.00" criteria for the back surface. (The distance portion back side curve would be -8.50 for plano.)

Part II
4. Example: assuming +3.00 with +2.50 add, the front curve would have to be ~+11.50 (which is -6.00 on the back at the bottom/-8.50 on the back at the top) which is a pretty steep base curve indeed.


Is this what you're saying?

[drk]

To risk taking it one step further:

THEN WHY DON'T COMPANIES MAKE FRONT SURFACE FREE-FORM PALS?

[optical24/7]

I addressed the deviation from best curve designs in my 2nd sentence in post #4.

I can only speak from personal experience:

I’m sph equivalent +1.75 with a 2.50 add. I have never experienced a total back side produced PAL as clear or comfortable as a hybrid design where part of the add is produced on the front. (Like Difinity or even Accolade Freedom). And every FF I’ve tried used around a 4.25 front. I also took many + patients out of FF’s. Of course, a lot of their positive reactions were due to Rx changes, but I had nothing but very positive remarks from those I switched. Now, my myopic pts were just the opposite. FWIW

[drk]

Why do I not know this stuff?