Question About freeform lenses and specifying base curve

**golfnut** · 05-22-2017, 10:19 PM

My lab is saying they can't specify a base curve on the auto III PAL. WE ARE DUPLICATI G AN OLD PAIR THAT IS A 3 BASE AND their computer is calling for a 4 base. Also, the lenses sent are VERY blue when looking on the side and her old were definitley not?? Have not had issues in the past?

**LENNY** · 05-23-2017, 12:05 AM

Are you duplicating RX also?
What they call 4 could be less, what you call 3 could be more :)
Blue? 1.67?

**lensmanmd** · 05-23-2017, 09:55 AM

Originally Posted by golfnut

My lab is saying they can't specify a base curve on the auto III PAL. WE ARE DUPLICATI G AN OLD PAIR THAT IS A 3 BASE AND their computer is calling for a 4 base. Also, the lenses sent are VERY blue when looking on the side and her old were definitley not?? Have not had issues in the past?

RX and material? What type of AR?

Compensated FF have a +/- 2D range in BC, depending on the ADD. ANSI has a +/-0.75D BC tolerance. Worrying about a 1BC differential is not worth it. I am assuming that this replication is from the file using the exact same frame, material and AR. Is this correct?

**Uncle Fester** · 05-23-2017, 10:41 AM

I've had this happen to me and was told Shamir every so often tweaks the program used to create the lenses. Therefore, the compensated pages sent with the finished order will not necessarily match .

Makes me go hmmm...

**Tallboy** · 05-23-2017, 11:45 AM

Originally Posted by lensmanmd

= ANSI has a +/-0.75D BC tolerance. Worrying about a 1BC differential is not worth it.

This. Much ado about nothing between a 3 and a 4.

I'd be more worried about the fact that one of the lenses is some kind of "blue blocking" resin and the other isn't, I have had patients not like new lenses as much as old because of this. Granted only 1 or 2, but it was enough to be aware of the possible issue.

**Hayde** · 05-23-2017, 12:49 PM

Are the patient's old lenses Autograph 3s as well? Identical match on the material & AR?

Contemporary freeform designs are Yakov Smirnoff PALs...they tell YOU base curve! When you're buying an "A3," you're buying the A3 Surfacing Equation...lock, stock, and barrel. If you tweak it, then it's not an A3 anymore and it won't get stamped as one. Same as most of the Digital PALs of the past several years.

If a prescriber (almost invariably behind the times) gives you a kneejerk imperative to match previous base curve, then you may be locked into an old mold in order to literally do it. If your patient is in an older PAL, fine.

If not, then the prescriber needs to provide some clarification. If there's an actual patient-centered reason to match an old base curve of a contemporary free-form, then this must be an interesting case. However, if you're just trying to oblige an arbitrary default text on the Rx pad to "MATCH BASE CURVE OF PREVIOUS PAIR," then yeah, read that as "TRUST THE FREEFORM DESIGN OF PREVIOUS PAIR." A call/email to the prescriber may or may not be fruitful if you're not sure. That's really a coin toss on how up-to-speed (or reasonable) the prescriber is. If the effort just confirms they're presumptively leaning into a default Rx policy instead of anything specifically relevant to the patient, then it was worth the effort. If so, as those above have said, the difference is most likely harmless and may even be ANSI-negligible to begin with.

At any rate, a prescriber whose mantra "base curve" is simply thoughtless habit needs to be inconvenienced with a little lens education if it otherwise means they're foolishly taking a successful free-form PAL wearer backwards to more archaic lens designs...whose old problems could be new again.

**tmorse** · 05-23-2017, 01:16 PM

Originally Posted by Hayde

Are the patient's old lenses Autograph 3s as well? Identical match on the material & AR?

Contemporary freeform designs are Yakov Smirnoff PALs...they tell YOU base curve! When you're buying an "A3," you're buying the A3 Surfacing Equation...lock, stock, and barrel. If you tweak it, then it's not an A3 anymore and it won't get stamped as one. Same as most of the Digital PALs of the past several years.

If a prescriber (almost invariably behind the times) gives you a kneejerk imperative to match previous base curve, then you may be locked into an old mold in order to literally do it. If your patient is in an older PAL, fine.

If not, then the prescriber needs to provide some clarification. If there's an actual patient-centered reason to match an old base curve of a contemporary free-form, then this must be an interesting case. However, if you're just trying to oblige an arbitrary default text on the Rx pad to "MATCH BASE CURVE OF PREVIOUS PAIR," then yeah, read that as "TRUST THE FREEFORM DESIGN OF PREVIOUS PAIR." A call/email to the prescriber may or may not be fruitful if you're not sure. That's really a coin toss on how up-to-speed (or reasonable) the prescriber is. If the effort just confirms they're presumptively leaning into a default Rx policy instead of anything specifically relevant to the patient, then it was worth the effort. If so, as those above have said, the difference is most likely harmless and may even be ANSI-negligible to begin with.

At any rate, a prescriber whose mantra "base curve" is simply thoughtless habit needs to be inconvenienced with a little lens education if it otherwise means they're foolishly taking a successful free-form PAL wearer backwards to more archaic lens designs...whose old problems could be new again.

Well said.

**merrymaker** · 05-23-2017, 01:35 PM

I've noticed Shamir's Glacier Plus ARC has a bluish-hue on the edges. Only apparent on a rimless job so far with no patient issues.

**golfnut** · 05-24-2017, 10:06 AM

Her original lenses were trivex with avance..new are the same with glacier ar and also reordered with avance. Rx in the 4 base eye is -.25-1.50. Her old rx was -.50-1.50. The old lenses clocked 2.75bc and on the work ticket it said 3bc. They are telling me its the material making it blue... old are definitely trivex and definitely not blue

**golfnut** · 05-24-2017, 10:11 AM

Also, the reason for matching base curve is pt came in to pick up and immediately felt like something was wrong...im not one who tries ti match old base curve just to do it. Different rx's call for different curves...the new rx is only .25 diff and pt is wondering why its si different ...im just trying to help her at this point.

**Hayde** · 05-24-2017, 10:15 AM

Thanks for the context! Was her old PAL an Autograph 3? If so, you may be on the right track & Uncle Fester's observation may be telling.

**golfnut** · 05-24-2017, 10:28 AM

Yes old pal and new are auto III 15 fixed trivex.

**Hayde** · 05-24-2017, 10:49 AM

Thanks Golf. Assuming you've already charted every measurement difference between old and new pair--then one more question, what was the axis change from old to new?

**lensmanmd** · 05-24-2017, 12:43 PM

And one more question.....what is the fit difference in frames from old to new?

**golfnut** · 05-24-2017, 05:54 PM

Frames are both silhouette, same style and shape...measurements duplicated. Axis changed 5 degrees

**lensmanmd** · 05-24-2017, 08:59 PM

Originally Posted by golfnut

Axis changed 5 degrees

SLH...oh my. 3 piece, I assume? Always SLH. Most of our 'non-adapts' occur with SLH. Delicate SOBs to adjust and keep in adjustment.

I'm thinking too much axis change for a 1.5 cyl. The refraction may be correct, but the patient just can't accomodate. Would you happen to know the variance of the compensated axis from previous to new RX? I would venture to say that the BC has little to do with your patient's discomfort.

**Kujiradesu** · 05-24-2017, 09:32 PM

Originally Posted by Hayde

At any rate, a prescriber whose mantra "base curve" is simply thoughtless habit needs to be inconvenienced with a little lens education if it otherwise means they're foolishly taking a successful free-form PAL wearer backwards to more archaic lens designs...whose old problems could be new again.

Agreed.

Once heard an old-timer optician remark: "When an OD writes MATCH BASE CURVE on an RX, it means: "Dont send him back to me, he's your problem now.""

**golfnut** · 05-24-2017, 09:43 PM

Originally Posted by Kujiradesu

Agreed.

Once heard an old-timer optician remark: "When an OD writes MATCH BASE CURVE on an RX, it means: "Dont send him back to me, he's your problem now.""

Originally Posted by lensmanmd

SLH...oh my. 3 piece, I assume? Always SLH. Most of our 'non-adapts' occur with SLH. Delicate SOBs to adjust and keep in adjustment.

I'm thinking too much axis change for a 1.5 cyl. The refraction may be correct, but the patient just can't accomodate. Would you happen to know the variance of the compensated axis from previous to new RX? I would venture to say that the BC has little to do with your patient's discomfort.

Her biggest complaint was the thickness and the fact that they were so blue compared to the old. The flatter base appeared much thinner ...if she didn't have the old to compare i wouldn't have a base curve issue i don't think...all of those things PLUS the difference in the steeper curve led her to dissatisfaction.

**Hayde** · 05-25-2017, 09:48 AM

If the axis changed towards the oblique, I wouldn't rule out lensman's supposition. But it sounds like her complaints are cosmetic rather than visual?

I found out yesterday that Shamir blanks are going 'blue blocking' officially on June 1. If the new lenses are reflecting more blue, then it wouldn't surprise me if your patient got one of the new ones a little earlier than advertised.

**lensmanmd** · 05-25-2017, 10:10 AM

Without knowing the frame dims, PD/SH, axis and add power, it is tough to calculate actual thickness. A quick DVO calculation using 50/32 with 31 PD @ 180, there is a 0.02mm ET difference between a 3BC and 4BC in TVX.

Safe to assume that add power increased and the sph decreased the same amount. This would add 0.5D to the overall thickness calculation based on near power, if all other measurements stayed the same. This in itself will increase overall thickness.