# Thread: PALs Bi-Convex at Near?

1. ## PALs Bi-Convex at Near?

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.

2. 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

3. Originally Posted by Lensman11
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?

4. 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.

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

6. Originally Posted by Kwill212
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.

7. 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/sho...l=1#post536029

Best regards,

Robert Martellaro

8. 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?

9. 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.

10. 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

11. Originally Posted by 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)
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

12. 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?

13. To risk taking it one step further:

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

14. 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

15. Why do I not know this stuff?

16. Much of this is no longer in OD curricula. Moved away to more medically oriented. Less CLs as well. During the time I was consulting for Paragon and CRT< we'd see ODs who had not read Fluoresceine patterns!

17. PALs was knew, when I waz in skule.

My new skul is Opteebord.

18. Excellent little ditty from Younger. Its copy righted.
https://www.camberlens.com/the_truth....aspx?langId=1

19. That's educational. I enjoyed that.

20. The vertically changing base curve of camber blanks is a big step forward in regards to base curve compromises.

In regards to your #5, IMO the reason you dont see many camber tech progressives "in the wild" is cost. Wholesale cost for a camber is a significant delta over any other progressive lens design.

21. Originally Posted by drk
To risk taking it one step further:

THEN WHY DON'T COMPANIES MAKE FRONT SURFACE FREE-FORM PALS?
Great question for progressive lens manufacturers.

Cost I'm sure is a factor but perhaps having the progressive design on the back surface is a greater advantage optically especially for plus RX's in regards to viewing thru the progressive corridor.

22. Thanks all for the responses! Been overwhelmingly busy the last... days, a happy problem to have.

Now, back on topic:

@ Robert - Most helpful, that's where I saw the source of my confusion, the diagram certainly helped more than the older Jalie diagram (for me, at least).

So, as far as my initial query is concerned, it's been resolved.

And in an effort to hopefully give back to the thread, from real fitting experience on real patients:

- I've dispensed PALs with near back curves that were Plano or close to it, and the near vision was tolerable... but only after discussion with the wearer who voiced concerns about cosmetics. Admittedly this isn't as huge an issue here in Asia, where moderate/high myopia is rampant. I've occasionally also deliberately requested a flatter blank for cases of hyperopes who were happy with the vision and not with the swim... they did notice a mild blurring or softening of the near vision, but were overall happier that they could comfortably wear the PAL full time (in cases where spherical front PAL were the only options).

- Among the supposed plus points touted to me for the Hoya ID or Camber blanks is that the two base curves of the spherical front surface (flatter on the top, steeper at the bottom), possibly allow for more balanced optics and cosmetics, since it's less about choosing a compromise SFSV blank, and the variable curve of the front surface allows for a slightly flatter fit with less effect on optics.

- Tangent, but related, the Varilux S/X series apparently use the reverse of this logic, by flattening the front towards the near zone to reduce magnification (and allegedly, therefore also reducing swim). This does tally with my experiences in the first point above.

- However, having to work and align both surfaces precisely introduces another possible margin of error, and presumably also increased risk of breakage during surfacing. I certainly notice much more delays on the ID series back when I dispensed it often than any SFSV design, as well as the Varilux S/X series.

Thanks for the discussion and sharing of knowledge, all, good stuff as always ��

23. Originally Posted by drk
To risk taking it one step further:

THEN WHY DON'T COMPANIES MAKE FRONT SURFACE FREE-FORM PALS?
I once asked a senior lab tech this question (a gentleman from a machinist background who worked production for many years, and whose optical knowledge, while formidable, was largely self taught through books... he even had his own copy of System of Opthalmic Dispensing).

His reply, if memory serves me correctly:

The default FF process involves blanks of fixed front spherical curvature, and just working the back of the lens

If you wanted to use the FF generator to grind a front surface PAL, you certainly could, as Hoya routinely does with their crossed-cylinder designs (ID and above).

However, since all FF blanks are by default plano-convex (with the front convex surface being the fixed reference curvature used to calculate and work the back surface), to work the progressive optics onto the front would mean also having to work the back to make it appropriately concave, therefore doubling the work (and the alignment issue between surfaces as mentioned prior).

Been years since that chat, and I might have mixed up some finer points there, but I believe that was it.

NB - He was also the first (and last) tech to show me a special kind of biconcave SV grind, where he took the thickest and flattest blank he had... Then combined working both surfaces and also some kind of lenticulation, to grind Rx ordinarily not achievable with available blanks. We do miss him now and then.

24. (OK, now here's where I try to pry Martellaro's trade secrets out of his gnarled, musician hands! Ssshhh!)

So...what do you do when you have a moderately high hyperopic presbyope?

Go back-surface FF PAL like everything else, and say "What, me? Worry?"
Go to esoterics like Camber?
Go back to front-surface-progression grinders?

I guess a lot would depend on the other distance lens power characteristics, such as higher or oblique astigmatism, or weird POW.

~No astigmatism issue---> Front surface grinder (with longer corridor) ---> ?
~Astigmatism issue or anisometropia issue ---> Camber, (Definity?)
~Weird POW, like wrap sun ---> maybe just lump it at near cause they ain't readin' much, anyway, with a wrap-inspired back surfaced FF design ---> (insert your favorite wrap design here, such as Attitude)

And where is the cut-off? +2.00?

25. Originally Posted by Elvis Is Alive
The vertically changing base curve of camber blanks is a big step forward in regards to base curve compromises.

In regards to your #5, IMO the reason you dont see many camber tech progressives "in the wild" is cost. Wholesale cost for a camber is a significant delta over any other progressive lens design.
Are you taking cost to the lab? or cost to the optician? I would disagree with the latter. Obvisouly there are cheaper designs(and there is no low tier Camber design) but I can get a Camber Steady+ for less(sometimes significantly) than branded top of the line lenses. Xseris, MyStyle, Auto Int., etc are all more. At the lab I use, they are less cost than most other even middle tier designs from other labs, like the Physio Drx, Comfort MAX, Auto 2, or Definity 2. I don't have a Zeiss price list to reference but I assume they are less cost than the Individual Wordsoup 3.

I think they aren't seen in the wild that often because some people just can't be pulled away from their sweet, sweet Kool-Aid.

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