My most recent head-scratcher. Help please!

[DanLiv]

Hi friends! Here's my most recent head-scratcher.

Made some Physio W3+ 1.74 in the following:
-13.25 -0.25 x015 +1.50 PD 30 seg 19
-10.50 -0.50 x100 +1.50 PD 30 seg 19

I used the same frame and duplicated measurements from previous lenses I made, which were same design and material.
Previous Rx:
-12.75 -0.75 x110 +1.25 PD 30 seg 19
-10.00 -0.75 x145 +1.25 PD 30 seg 19

Customer definitely sees better with new Rx, but notices even better clarity when tilting head back to look slightly into the intermediate area. I confidently put +0.25 and +0.50 flippers over thinking she's just overminused, but nope, does not like that. Doc re-refracted and nope she rejected plus there too. Gave her -0.25 and -0.50 flippers, that makes DV a bit sharper, but still even better if tilting back. I verified fit and lens power and prism hoping to find some issue, but none. It seems like she's struggling to access more plus, but giving that plus any other way doesn't help.

The only thing I wonder about is has the increased minus and the increased add now created just enough power delta in the corridor that the PRP being 4mm below the pupil is causing her to notice off-axis issues when looking straight ahead? It sure seems like when she claims best vision she is looking straight through PRP. Could that really be it? I've certainly fit higher minuses with higher adds in progressives before without this issue, so it could just be idiosyncratic to her.

I'd hate to simply raise the seg height and cross my fingers, I'd rather have more sound reasoning to any changes. Any other ideas I'm missing?

[AndyOptom]

Unsure about how the Rx compensation on this particular Varilux Physio design is done, is it possible the sizable axis changes could have led to this patient feedback?

Also, given the high Rx, any significant difference in the bevels between both pairs of lenses? I've had patients ordering multiple pairs of lenses before where (due to lack of specified bevel placement on the order form) both pairs came back with different bevel placement and their corresponding difference in patient feedback.

[finefocus]

Total minus is similar, but wild axis changes, especially OD. Posture changes produce angle changes which simulate cylinder axis changes.
If the old axes were wrong, and the new ones correct, patient will adjust. Meanwhile, why not adjust with a bit more retro to mimic what
the posture changes induce?

[drk]

Axis change looks worse than it is...low amounts of cyl pop up like rainclouds and disappear (although I always think about how it will affect the patient).

The only thing I wonder about ... the PRP being 4mm below the pupil is causing her to notice off-axis issues when looking straight ahead? It sure seems like when she claims best vision she is looking straight through PRP.

Nope, you've got it, right here. I experience this myself, I believe...with high index/high power lenses the optics degrade so quickly off the optical center that you can notice even if you're looking 2 mm above the prism reference point.

I would indeed raise the fitting height and that's all you can do. (Well, I did recently remake to CR-39 for myself just to prove the point that it's the high index optics...and it worked. You could jigger the index down to 1.6.)

Plus there's some non-insignificant anisometropia and you never know what people interpret as "blur" when it's kind of "double". (But they'd complain first in the near zone, of course, so I wouldn't put too much stock in that consideration. But raising the PRP will make a positive impact on this issue as well.)


Just to be sure I'm not full of it, the prism reference points are the best optics, and the fitting height is usually about 2mm above that (and of course they ask us to read the distance power even further above that, I believe because they don't want progressive power change blur in the stop of the lensometer).

[Uilleann]

Make a switch to 1.67 perhaps? Also...cataracts / diabetic?

[AndyOptom]

Also, just noticed something that may be related:

Am unfamiliar with this particular variant of Varilux Physio (since locally here it's just the default Varilux Physio and the Varilux Physio Fit for POW compensation, the default with fixed corridors and the Fit with variable corridors unless specified)... are the corridors between both pairs different? I see no mention of this, and indeed with some previous patients, I've encountered somewhat similar issues faced from the same patient swapping between multiple pairs with Physio short or longer corridors. Those prior cases of mine happily were easily resolved with some frame adjustments, but then again those did not feature any Rx changes as in your case here.

[Robert Martellaro]

Also, given the high Rx, any significant difference in the bevels between both pairs of lenses? I've had patients ordering multiple pairs of lenses before where (due to lack of specified bevel placement on the order form) both pairs came back with different bevel placement and their corresponding difference in patient feedback.

VD delta in right of 2mm is a smidge more than 0.25 D. So maybe, but Dan did over-refract ruling out power issues, but still might be a contributor.

Axis change looks worse than it is...low amounts of cyl pop up like rainclouds and disappear (although I always think about how it will affect the patient).

Yup, cylinder change is likely inconsequential. There is a sphere equivalent bump in the left of -0.375, with no change in right. Power changes in one eye only always make me feel a tad squirrely for a short period of time.

Nope, you've got it, right here. I experience this myself, I believe...with high index/high power lenses the optics degrade so quickly off the optical center that you can notice even if you're looking 2 mm above the prism reference point.

4mm induces 13.50 x .4mm = 5.4∆, enough to be noticeable, especially if there are other errors and aberrations. Not new though.

I would indeed raise the fitting height and that's all you can do. (Well, I did recently remake to CR-39 for myself just to prove the point that it's the high index optics...and it worked. You could jigger the index down to 1.6.)

I wouldn't raise it above the corneal reflex. Good to see you've hopped on the CA train- it's the real deal. Unfortunately Varilux is N/A above -12 D. Shamir can do it though.

Plus there's some non-insignificant anisometropia and you never know what people interpret as "blur" when it's kind of "double". (But they'd complain first in the near zone, of course, so I wouldn't put too much stock in that consideration. But raising the PRP will make a positive impact on this issue as well.)

Add a little BU prism? Also, the new Rx left eye has a -0.375 bump in sphere equivalent; she was getting a little light monovision in the old Rx. More of an issue with absolute/advanced presbyopes though.

And then there's that add power change...

Dan, You've probably already done so but she should wear for 10 days or so then reevaluate. Any chance you have an identical frame for the old lenses to compare? Good luck, Robert.

[drk]

https://www.2020mag.com/article/drop...to%20the%20PRP.

[drk]

The phenomenon is noticeable by the patient, and it's not about comparing the old glasses to the new ones, so we can just ignore all the old information.

Increasing panto gives an artificial increase in cylinder axis 180 that may mask the effect a little (but isn't an ideal situation). It lowers the optical axis, though and will increase vertex distance at the top of the lens, so it works against you as well. May be worth a try to see if the patient likes it.

Decreasing panto will give the effect of raising the PRP and shortening the top vertex distance and the patient may like that, too.

As always, with the minus fiends, smashing the vertex distance to as close as possible is always a winner.

I don't think any of the above are the "right" solution, but are easy solutions based on POW!

[Robert Martellaro]

https://www.2020mag.com/article/drop...to%20the%20PRP.

Most manufacturers have adopted 4mm to give the market a standard for less confusion between lab and dispensing opticians. The drop has no significance visually except for aniso Rxs where a 6mm drop (Zeiss?) might induce too much VI on the distance gaze.

[Robert Martellaro]

The phenomenon is noticeable by the patient, and it's not about comparing the old glasses to the new ones, so we can just ignore all the old information.

The OP said it was a lenses only job, the frame parameters are identical for both.

As always, with the minus fiends, smashing the vertex distance to as close as possible is always a winner.

The only problem is the client- moving the bevel back helps make the lens look thinner, especially with plastics, using a super hi-index material looks thinner also. When given a choice, many clients go for the cosmesis, sacrificing quality of vision. So it goes, -Robert.

[drk]

Here's a mind-number.

Let's say you decide to bevel a -10.00 in a thin metal eyewire. Let's say it's a 10mm thick lens for ease of discussion.

Let's say you would normally bevel the lens so that the front of the lens doesn't protrude...the bevel is 1 mm from the front, so 9 in back. That would be the "right vertex" for the Rx, I'm supposing.

Let's say instead you decide to bevel a -10.00 with a 1/3-2/3 in a thin metal eyewire because you think the cosmesis is preferable that way. So you have about 3 mm forward, and about 6 mm backwards. But let's say that you're astute, and realize you're moving the lens forwards 2 mm by this process, and you need to compensate the power for the vertex distance change. So you do the calculations, and add (let's just say) -0.50 more minus to compensate.

Question: does the addition of more minus power, which makes the lens thicker, offset what you were trying to do: that is, improve cosmesis?


My initial answer: it wouldn't add that much more minus, so it's still worth it.

[Prentice Pro 9000]

https://www.2020mag.com/article/drop...to%20the%20PRP.

"The drop of a progressive lens is not in and of itself a design element, and it is not directly related to either the progressive profile or placement of unwanted cylinder power. It is, quite simply, the lens designer’s chosen placement for the prism reference point in relation to the fitting cross"

I hate this. I feel like I've never gotten a straight answer about why lens designers would choose to go with one drop over another. This article makes it seem as if they just kind of make it up or that it's an artistic choice. Oh and by the way, there's no reference to any lens designers telling her this or another article or white paper to quote from. She just made it up.

[drk]

I'm going to step in it, here. I'm kind of locked into "old school" thinking, so someone will clean my mess up.

Let's talk basic, non-individualized SV: If we were to put a "fitting cross" on a SV lens (which is actually a felt-tip pen dot for most of us) we'd order the "prism reference point" 3-4 mm below it. That's because the eye looks down to read, in addition to straight-ahead, so we tilt the lens bottom in by 5-10 degrees to maintain equidistant vertex, and we have to drop the "prism reference point" (optical center) in order to have the optical axis of the lens co-incident with the eyes' ephemeral center of rotation.

So what's the "drop" in a SV? 3-4mm. Sound familiar?

So, in an old-school PAL, the same principle applied. I believe. It's "equal vertex in S.A and down gaze ---> 8 degrees pantoscopic tilt ---> 4 mm drop of the O.C./PRP."

Now I'm told in modern free-form complex surfaces that all that is blown up. Maybe I'm wrong. That jibes with her statement: "The drop of a progressive lens is not in and of itself a design element, and it is not directly related to either the progressive profile or placement of unwanted cylinder power. It is, quite simply, the lens designer’s chosen placement for the prism reference point in relation to the fitting cross"

[AndyOptom]

I feel like I've never gotten a straight answer about why lens designers would choose to go with one drop over another. This article makes it seem as if they just kind of make it up or that it's an artistic choice.

IIRC, prior to free form designs, when it was all molded front surface progressives, then the location of the PRP was more relevant for various reasons, including pantoscopic tilt calculations etc as drk mentioned.

So if we are asking what advantages one drop may have over the other, you'd have to think more in terms of the limits of progressive lenses back then.

E.g. per some old posts by the late (and great) Daryl Meister, the Zeiss 6 drop helped offset some prism effects in the near zones (since the geometric center of the lens was closer to the midpoint of the corridor length). But this would then cause some prism effect on the distance zone.

AFAIK the designs currently used for free form progressives work around those traditional considerations, there might be a post about it by lensgrinder around somewhere IIRC.

[Prentice Pro 9000]

I'm going to step in it, here. I'm kind of locked into "old school" thinking, so someone will clean my mess up.

Let's talk basic, non-individualized SV: If we were to put a "fitting cross" on a SV lens (which is actually a felt-tip pen dot for most of us) we'd order the "prism reference point" 3-4 mm below it. That's because the eye looks down to read, in addition to straight-ahead, so we tilt the lens bottom in by 5-10 degrees to maintain equidistant vertex, and we have to drop the "prism reference point" (optical center) in order to have the optical axis of the lens co-incident with the eyes' ephemeral center of rotation.

So what's the "drop" in a SV? 3-4mm. Sound familiar?

So, in an old-school PAL, the same principle applied. I believe. It's "equal vertex in S.A and down gaze ---> 8 degrees pantoscopic tilt ---> 4 mm drop of the O.C./PRP."

Now I'm told in modern free-form complex surfaces that all that is blown up. Maybe I'm wrong. That jibes with her statement: "The drop of a progressive lens is not in and of itself a design element, and it is not directly related to either the progressive profile or placement of unwanted cylinder power. It is, quite simply, the lens designer’s chosen placement for the prism reference point in relation to the fitting cross"

for sure, I get it being at a 3-4 since that's what most panto calls for. I guess I'm stumped when it goes beyond that range into the 0s,2s,6s.

[Prentice Pro 9000]

IIRC, prior to free form designs, when it was all molded front surface progressives, then the location of the PRP was more relevant for various reasons, including pantoscopic tilt calculations etc as drk mentioned.

So if we are asking what advantages one drop may have over the other, you'd have to think more in terms of the limits of progressive lenses back then.

E.g. per some old posts by the late (and great) Daryl Meister, the Zeiss 6 drop helped offset some prism effects in the near zones (since the geometric center of the lens was closer to the midpoint of the corridor length). But this would then cause some prism effect on the distance zone.

AFAIK the designs currently used for free form progressives work around those traditional considerations, there might be a post about it by lensgrinder around somewhere IIRC.

So if that's a thing, then that kind of blows up what this article was saying about the drop just being kind of a whim of the designer with no real effect on the other attributes of the lens.

[DanLiv]

given the high Rx, any significant difference in the bevels between both pairs of lenses?

Yes actually. This is a 2/3 bevel while the previous lenses were front bevel. However, it is a small rectangular frame and there is minimal thickness top and bottom to dramatically change the vertex point. I haven't measured it, but if it were as much as 2mm increased vertex we'd be losing a little over 0.25D minus power, and I would expect that to be easily teased out by the -0.25/-0.50 flippers, but just giving here minus did not rectify the issue of disparity between looking through the different parts of the lens.

with high index/high power lenses the optics degrade so quickly off the optical center that you can notice even if you're looking 2 mm above the prism reference point. I would indeed raise the fitting height and that's all you can do. You could jigger the index down to 1.6.)

Make a switch to 1.67 perhaps? Also...cataracts / diabetic?

No cataracts or diabetes, customer is 49. She does have keratoconus, however it's well-managed with scleral lenses and she's had a stable DV Rx since 2008. I've also had her in 1.74 since 2008. 1.67 would negligibly improve aberration (maybe JUST enough to make the difference for her?), 1.60 would do a lot better but I think the thickness would be off-putting.

Thank you all for the counsel. I was concerned there was some obvious issue I was missing, I'm glad to know it wasn't simple and that my instincts were going in the right direction. I'm going to raise the seg 1mm, and play with vertex and power to net her an extra -0.25, because I want to keep the 2/3 bevel it looks much better than the front bevel.

Question: does the addition of more minus power, which makes the lens thicker, offset what you were trying to do: that is, improve cosmesis?

You're right the additional power has a negligible impact on thickness. The percentage of power change due to fit is small, I don't think anyone could tell a difference in thickness on visual inspection alone between -10.00 and -10.50. And if you're ordering fit compensated designs for these powers, you're already getting those higher power lenses anyway!

[Robert Martellaro]

No cataracts or diabetes, customer is 49. She does have keratoconus, however it's well-managed with scleral lenses and she's had a stable DV Rx since 2008. I've also had her in 1.74 since 2008.

There you go. Maybe irregular astigmatism, probably higher order aberrations (coma etc.), becoming more noticeable with the stronger add. You could try a different PAL design, but ultimately segmented or separates should be on the table.

Hope this helps,

Robert Martellaro

[AndyOptom]

Yes actually. This is a 2/3 bevel while the previous lenses were front bevel. However, it is a small rectangular frame and there is minimal thickness top and bottom to dramatically change the vertex point. I haven't measured it, but if it were as much as 2mm increased vertex we'd be losing a little over 0.25D minus power, and I would expect that to be easily teased out by the -0.25/-0.50 flippers, but just giving here minus did not rectify the issue of disparity between looking through the different parts of the lens.

<snip>

Thank you all for the counsel. I was concerned there was some obvious issue I was missing, I'm glad to know it wasn't simple and that my instincts were going in the right direction. I'm going to raise the seg 1mm, and play with vertex and power to net her an extra -0.25, because I want to keep the 2/3 bevel it looks much better than the front bevel.

Sounds like a plan, and if all else fails, maybe reconfirm the axis changes (since she's after all a keratoconus patient) :)

Personally, for all higher minus cases passing through my dispense, if possible I compare the current Rx + POW + bevels, and adjust accordingly. My preference in dispensing for these cases is often a 1/2 bevel, which works wonders to hide thickness in even 1.67 due to the popularity of plastic frames here in Asia (coincidentally also where we see many high minus cases).

I also find that 1/2 or 2/3 bevels often allow some wiggle room for nosepad adjustments etc.

So if that's a thing, then that kind of blows up what this article was saying about the drop just being kind of a whim of the designer with no real effect on the other attributes of the lens.

For simpler prescriptions, I'd be inclined to agree, this is a reasonable rule of thumb nowadays.

However, for aniso prescriptions and progressive designs with binocular prism balancing (e.g. Master series lenses by Nikon, Multigressiv and higher by Rodenstock, etc), according to the lens reps these prism balancing calculations use the PRP as the reference point for calculations etc to try and reduce vertical imbalance.