PALs Bi-Convex at Near?

[AndyOptom]

For segmented multifocals I'd agree, that's also how I was taught to verify them, convex side facing down. Reading distance and near Rx separately, then deducting the difference to get the add.

The lenses being discussed here are progressives, however, and not segmented multifocals.

For progressives, be it manual or digital lens meters, with the old front surface designs I never saw issues verifying the add even with the concave side facing downwards towards the lens meter stop. The distance and near Rx consistently could be verified as such, even if the method was wrong.

When everyone started moving progressive optics to the back surface, the add often read weaker than even the compensated/recalculated/etc modified Rx the labs provided for verification.

Explanation given by the labs was that it was not a production defect. So we were told to flip the lens and verify the back surface progressive as if it was a segmented multifocal, for the reason as described in my first post.

Doing so yielded verified Rx within tolerances, and wearers had good vision + feedback with the lenses on dispense, therefore I left it at that.

[Lensman11]

The reason you turn a segmented lens around to check the power of the add is this eliminates the thickness of the lens which adds plus power to the seg. When you do this you must read the distance as well as the seg reversed and the difference is the power of the seg. The explanation from Andy makes no sense to me.

[drk]

I think that's the way to verify any front surface multifocal, even segmented. Right? Not that anyone does that. I don't think that's proof.

[AndyOptom]

OK, to update this thread and cross-reference it, Essilor designs are front surface progressives. https://www.optiboard.com/forums/sho...ace#post574303

Might be a regional/geographic variation in product availability, but as far as my local reps tell me, Essilor/Nikon lenses in Southeast Asia all have the progressive optics on the back surface, with spherical front surfaces (except for the Varilux X and formerly, S series).

The apparent proof to their claim is when we verify the near Rx of received lenses, the addition is often weaker than even the compensated Rx provided for verification... they tell us to flip the lens such that the front surface will face the lens meter stop, and then to verify the near Rx. This explained to me as being possible since the progressive optics being on the back may encroach within the aperture of the lens meter stop, and therefore affect the reading.

So far, it's worked out as claimed, and they've been good with remakes for those which did not pass verification even that way.

[drk]

OK, to update this thread and cross-reference it, Essilor designs are front surface progressives. https://www.optiboard.com/forums/sho...ace#post574303

[Robert Martellaro]

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

Depends on their history- Rx, BC/thickness, material, etc. I've used Multigressiv, Varilux, Definity, Ind. 2I, etc.

Robert

[Kwill212]

Depends on the lab. I would hope and assume labs are only using top end designs with Camber blanks... ...US Optical's CamberHD is cheaper than alot of their Varilux lines, I'll give you that (gives me slight pause on the quality of the design to be honest).

It's not like they can take a Camber blank and slap any old lens design on it. A Camber Steady from one lab is going to be the same as Camber Steady from any other lab. They all have to pass the same certifications with IOT to produce the lens designs.

Cherry has IOT's Camber Steady in the same (top) price tier as Varilux S/X, so no difference there.

I know, which is bonkers if you ask me. (PM me if you want to discuss more on IOT lens price.)

[Elvis Is Alive]

Depends on the lab. I would hope and assume labs are only using top end designs with Camber blanks.

Cherry has IOT's Camber Steady in the same (top) price tier as Varilux S/X, so no difference there. US Optical's CamberHD is cheaper than alot of their Varilux lines, I'll give you that (gives me slight pause on the quality of the design to be honest). Several others are significantly higher.

[Kwill212]

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.

[drk]

(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?

[AndyOptom]

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.

[AndyOptom]

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 ��

[Elvis Is Alive]

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.

[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.

[drk]

That's educational. I enjoyed that.