Aspherics and power variation in periphery.

[Hayde]

You're right. I should have said "When considering an aspheric for cosmetics, always go FFSV".

Likewise, I'm now saying "When you have a moderately astigmatic or higher-powered lens, try to go FFSV, for better optics."

But maybe we should get to the point when we consider SV lenses totally differently, as you thought I was saying!

What if...

...similarly to the way we see a presbyopic Rx and immediately think of our favorite PAL design (only defaulting to segmented if PALs are contraindicated)...

...we looked at a SV Rx and immediately think of our favorite FFSV brand (only defaulting to a spherical lens if FFSV is overkill or cost-prohibitive)?

LOL I may be the wrong guy to ask, since I don't automatically think PAL when I see a presbyope Rx. (My clientele base is rather old fashioned and FTs remain a welcome, first tier solution to many a Gordian knot.)

My only real concern with your take here is that it seems to rule traditionally surfaced aspherics obsolete. If Tallboy is looking at price sheet that says traditionally surfaced aspherics are the new 1.56 mid-index destined for economic obsoletion, I get it. But my price sheet doesn't look like that yet. And no lens rep I've met yet has tried to convince me that digital SV replaces traditional aspherics altogether. They might say so if asked, but so far they don't seem to show up prepared to hype FFSV. My hunch is that digital tech can be a great distortion fighter, and PALs are great big plains of distortion. Aspheric SV lenses aren't...and so until powers really climb up, there's just not that much for digital tech to fix.

My own rule of thumb is that if I have to spend more than a second calculating in my head if a vertex measurement is warranted (or it was an obvious 'yes' already,) it's time to present Digital SVs as an option. To my (admittedly statistically-lacking) patient feedback, this is when the POW measurements make an appreciable difference in the vision a FF produces versus a traditional aspheric. If I'm not giving the FF enough credit, I'm receptive to some customer satisfaction studies enjoying some good rigor.

What do you have for me, Pete Hanlin? :)

[Robert Martellaro]

That makes a ton of sense (where the meat is I mean). I've heard that Seiko has a front and backside aspheric/atoric lens, I wonder if the Zeiss 1.74s I get from Germany/Japan that are so crazy flat but seem to work are utilizing that tech.

Zeiss might be surfacing the front on their very high plus.

Thanks for the 4mm to the prism reference point info, I always thought the PRP location didn't matter - but I guess I see if there was a major imbalance there could be issues. I almost exclusively use IOT these days for digital sv, some shamir also - both are OC on the PRP. Whatever works.

Your welcome. Have you compared their performance in front of your own eyes, using the same frame, Rx, material, etc?

I would love Barry Santini to chime in on this thread because I know he has shown an affinity for Zeiss over Shamir digital lenses- and one of his sticking points was the location of the OC on the PRP instead of 4mm above like Zeiss.

It's the fitting point that's 4mm above. Shamir does allow about 2∆ of prism thinning, up for minus and down for plus.

...we looked at a SV Rx and immediately think of our favorite FFSV brand (only defaulting to a spherical lens if FFSV is overkill or cost-prohibitive)?

I think I'm paying less for some the FFSV compared to semifinished. Just don't use it for moderate to high values of prescribed prism, non-adapt is likely.

I wish we could input the work distance for FF SVNO. I know Zeiss does for their PALs. Brent (Lensgrinder), if you're reading this, is it possible with Zeiss SV?

Pause slides as necessary to read contents, enjoy:

You whipped this up just now? Nice going.

Best regards,

Robert Martellaro

[UtilityDave]

Bingo! The digital lenses may advertise superiority on both counts, but I'd hesitate to sell them as such unless powers are rather strong and you're very proficient at VWT measurements (aka POW measurements.) Digital in and of itself doesn't make anything better--it's the 'secret sauce' computations those digital products are using that may (or may not) be worth the price bump.

Could you expand on this?

I am trying to figure out which patients would be good candidates for digital designs, and which digital designs do what (Which ones improve thinness only, which ones improve periphery optics, which ones are just calibrated for position of wear, and how can I find the difference?)

The training I got in relation to this was "Digital designs don't work, don't bother with them" but I suspect it is more nuanced than that, and I would like to investigate it more.

Thank you.

[optical24/7]

Pause slides as necessary to read contents, enjoy:

Excellent MO! Sometimes explaining what aspheric/atoric lenses are accomplishing gets complicated, not easy to understand explanations. Short, concise and to the point...Once again, excellent!

[drk]

I wonder, even in low power lenses, if it's worth it to have the "exact" lens form via digital, vs. being shoehorned into a tranche of pre-fab base curves via traditional surfacing?

I guess it's most likely to be power-dependent like everything else; a -1.00 on a +6.00 vs. on a +5.70 (made up example) my not register for the average human being.

I think we could use Darryl's Optical Analyzer software to get a better feel for it.

What do you people think?

[Hayde]

Could you expand on this?

I am trying to figure out which patients would be good candidates for digital designs, and which digital designs do what (Which ones improve thinness only, which ones improve periphery optics, which ones are just calibrated for position of wear, and how can I find the difference?)

The training I got in relation to this was "Digital designs don't work, don't bother with them" but I suspect it is more nuanced than that, and I would like to investigate it more.

Thank you.

DrK has been politely trying to get a simple answer out of me, too--I'm just too stubborn.

"Digital designs don't work, don't bother with them" is the complete opposite of training. It has something to do with management inspired by Sunzi and nothing to do with optics or ophthalmic care. Sorry you're in that situation. Measurements for digital lens products are just like any measurement in optics...very easy to do wrong. When digital data inputs are wrong, a very sensitive calculation can turn a digital lens into a train-wreck very quickly. Some people definitely shouldn't bother with them...but that doesn't mean they don't work.

This thread so far has some difference of opinion when digitals are optimal for SV patients--it could be that Robert is right where I'm wrong. (In fact, that's a pretty darn good bet at all times if you're the gambling sort.) But until Pete plops down some internal data that slaps me in line, I'm going to tell you that the odds of digital SVs providing an appreciable improvement in 'quality of vision' over traditionally surfaced aspherics breaks in your favor for patients of powers >4.25D. Specifically, I'm talking about the Essilor 360 Fit. The other guys will have other choices based on more experience than I have.

[Tallboy]

Zeiss might be surfacing the front on their very high plus.

Your welcome. Have you compared their performance in front of your own eyes, using the same frame, Rx, material, etc?

It's the fitting point that's 4mm above. Shamir does allow about 2∆ of prism thinning, up for minus and down for plus.

I think I'm paying less for some the FFSV compared to semifinished. Just don't use it for moderate to high values of prescribed prism, non-adapt is likely.

Doh, I know that it is fitting height (or as I instruct my employees - always Take the dang Pupil Height, if you want to know where to put the OC I'll teach you) I was just entering stuff into DVI and as you know OC means fitting height in pretty much all lab software so... yeah I said the wrong thing

As far as FFSV with prism I actually almost exclusively use the Shamir Spectrum SV for all prism jobs I do and have spectacular results. I was told by a Shamir engineer not to use the Autograph on such jobs (because the compensation program doesn't handle the prism well) and try the Spectrum, it has worked out great for our patients and clients.

My own rule of thumb is that if I have to spend more than a second calculating in my head if a vertex measurement is warranted (or it was an obvious 'yes' already,) it's time to present Digital SVs as an option. To my (admittedly statistically-lacking) patient feedback, this is when the POW measurements make an appreciable difference in the vision a FF produces versus a traditional aspheric. If I'm not giving the FF enough credit, I'm receptive to some customer satisfaction studies enjoying some good rigor.

Even with a perfectly fit vertex distance its about custom asphericity/atoricity as opposed to set radial asphericity/atoricity.

I have about a -5.50 sph and -0.50 cyl in each eye. Give or take a quarter diopter. I feel like I am right on the edge of someone that can "notice" the difference. The two things I notice in my IOT sv digital (raypath technology mumbo jumbo) was when I wore my 1.67 aspheric Crizal or Zeiss lenses things looked less "3D", I felt like I have better depth perception in my spherical front FFSV lenses, there is probably about 20-30% more "perfectly clear" peripheral vision also, I can almost see all the way to the edge of my 55 eye frames (about 3mm decentration)

I have a hard time deciding where my "cutoff" is for reccomending them, so I just use them if I need them surfaced or if my gut tells me too.

[Hayde]

Even with a perfectly fit vertex distance its about custom asphericity/atoricity as opposed to set radial asphericity/atoricity.

I have about a -5.50 sph and -0.50 cyl in each eye. Give or take a quarter diopter. I feel like I am right on the edge of someone that can "notice" the difference. The two things I notice in my IOT sv digital (raypath technology mumbo jumbo) was when I wore my 1.67 aspheric Crizal or Zeiss lenses things looked less "3D", I felt like I have better depth perception in my spherical front FFSV lenses, there is probably about 20-30% more "perfectly clear" peripheral vision also, I can almost see all the way to the edge of my 55 eye frames (about 3mm decentration)

I have a hard time deciding where my "cutoff" is for reccomending them, so I just use them if I need them surfaced or if my gut tells me too.

Thanks for the personal anecdote, Tallboy! I really find that helpful. Is that 3mm decentration for one eye or combined OU? What material is your IOT?

[Tallboy]

Hayde n the pair I was talking about (A Ted Baker that became my "experiment pair") It was 1.67 IOT and 1.74 Zeiss dual aspheric stock lens and Crizal 1.67 aspheric stock lens. Both the Crizal and the IOT lenses were trans gray, zeiss was clear. Decentration is 3 in the OD and 2.5 in the OS.

I've also succesfully worn spherical poly before because I'm a cheap ******* lol.

The pair I have now is 1.60 IOT SV and for what its worth the 1.6 feels a little "brighter" vision, and its nice. The 1.67 was definitely thinner though. There is something to the depth perception thing, but its a phenomena I've noticed in regular spherical lenses also (aspheric always feels like it flattens out my vision, like things seem further away kind of)- however the IOT FFSV gives me that non-flat feeling with the edge to edge clarity of the Zeiss dual aspheric 1.74, only slightly more so.

And I am NOT the guy who pushes FFSV on everyone.

[drk]

I would like to know, Mr. Tall, if you can compare your essentially spherical moderate myopic Rx vision in any kind of FF with your vision in a spherical lens. (Except maybe cheap poly, although I'm a huge poly fan.)

[Robert Martellaro]

This thread so far has some difference of opinion when digitals are optimal for SV patients--it could be that Robert is right where I'm wrong. (In fact, that's a pretty darn good bet at all times if you're the gambling sort.) But until Pete plops down some internal data that slaps me in line, I'm going to tell you that the odds of digital SVs providing an appreciable improvement in 'quality of vision' over traditionally surfaced aspherics breaks in your favor for patients of powers >4.25D. Specifically, I'm talking about the Essilor 360 Fit. The other guys will have other choices based on more experience than I have.

I think were pretty much in agreement WRT FFSV.

Here's another example. Rx -5.00 OU, client wants a 'tall' frame that places the pupil height 7mm above the datum line, but the pantoscopic tilt cannot be increased to more than 2 degrees because the bottommost part of the frame/lens will rest on the cheek. Solution- use a POW design to reduce the induced aberrations from an improper alignment of the lens with the eye's center of rotation.

Doh, I know that it is fitting height (or as I instruct my employees - always Take the dang Pupil Height, if you want to know where to put the OC I'll teach you) I was just entering stuff into DVI and as you know OC means fitting height in pretty much all lab software so... yeah I said the wrong thing

No worries. When I make another typo you let me know and I'll fix it.

As far as FFSV with prism I actually almost exclusively use the Shamir Spectrum SV for all prism jobs I do and have spectacular results. I was told by a Shamir engineer not to use the Autograph on such jobs (because the compensation program doesn't handle the prism well) and try the Spectrum, it has worked out great for our patients and clients.

I take it one step further and use spherical Trivex, especially for near only, for moderate to high prism powers, even though it costs me more to do so. I feel I have more control over the optics, especially with higher refractive powers, even with the Spectrum being pretty much a 'dumb' lens.

I have a hard time deciding where my "cutoff" is for reccomending them, so I just use them if I need them surfaced or if my gut tells me too.

It can be a tough call. I think it's best to error on the side of more refinement.

Best regards,

Robert Martellaro

[Barry Santini]

It's not "OC on the PRP."
It's "MRP on the PRP."

[drk]

I've been told "MRP" is out of vogue.

Now, it's "<insert term I forgot here>".

[Tallboy]

I would like to know, Mr. Tall, if you can compare your essentially spherical moderate myopic Rx vision in any kind of FF with your vision in a spherical lens. (Except maybe cheap poly, although I'm a huge poly fan.)

Well I've only worn truly spherical lenses of two types since I've been in the industry. Tons of poly (mostly Somo) and 1 pair of CR39 Polarized KBCO lenses that I made around 11 years ago. Before that I had a pair of Spectralite that lasted me around 10 years or so? Before I was in the business. Before that I guess I was in CR39 lenses my mom bought me (I played football and basketball wearing them, its amazing I'm not blind I guess...).

I had been in aspheric for a looong time before I did the spherical CR39 polarized - I felt fishbowl for almost an entire day, then it got better, then they were my favorite sunglasses vision wise until my RX changed.

The spherical Poly stuff I've worn a lot of, and I saw fine out of it, but there was definitely much more bending of images at the edges, I didn't get the "flat" feeling out of them. The flat feeling isn't a bad one, just a noticeable difference in how the lenses are working. Thats my story and I'm sticking to it. I should get bored and cut some CR39 for myself.

[MakeOptics]

https://makeoptics.com/harrychiling/...ace-calculator

Play with powers, curves, and asphericity to your hearts content. Enjoy.

[Robert Martellaro]

It's not "OC on the PRP."
It's "MRP on the PRP."

I've been told "MRP" is out of vogue.

Now, it's "<insert term I forgot here>".

They're synonyms.

Prism Reference Point is the modern term for Major Reference Point. From the Germans circa 1990's.

Prismenbezugspunkt.

Best regards,

Robert Martellaro

[lensgrinder]

A molded aspheric lens is rotationally symmetrical, meaning that if you measure from the center outward to a point and measure again from the center to the same distance but opposite the other point they will measure the same curve. The lens flattens or steepens at the same rate from the center. If you drop a rock in a pond the waves that are produced are the same or symmetrical.
You will want to have this curve change to offset for the aberrational astigmatism and increase in mean power that is being produced as your eye rotates away from the center. If you have an astigmatism you will only be able to control one meridian of the lens.
The amount of asphericity will depend on what the designer has chosen to control(i.e. they may want to reduce mean power by increasing astigmatic error or reduce astigmatic error all together and not worry about mean power). The goal of the lens design is to have all of the light fall on the far point sphere. This is a sphere whose radius is equal to the Center of Rotation plus the vertex distance minus the back vertex focal length.
As an example if you have a +3.00 with 30 degrees of eye rotation placed on a material of 1.5 in an 8.00 base. The astigmatic error is 0.12 D and the overall mean power is 2.92 D. We can reduce this error by increasing the base curve to roughly 9.5, by doing this we eliminate the astigmatic error and decrease the mean power error to 2.84. It might be difficult to place a 9.50 D BC in some frames so we use an aspheric lens. We can use a 4.50 BC with an aspheric design that keeps the same mean power error and eliminates the astigmatic error as the 9.50 BC did.
Even best form lenses will produce some form of error, for example if you want to reduce oblique astigmatism you will have some mean power error, if you decide to reduce mean power error you will be left with some oblique astigmatism. Most "best form" lenses set out to reduce tangential error which is to have the back vertex power equal the tangential error.
So any molded product will be molded with known averages as is the case with best form. Best form lenses use an Rx range, where each base curve will fit a particular Rx range and use spherical equivalent when cylinder is introduced. In can be shown through Tscherning's Ellipse that every back vertex power requires its on BC, but this was/is not practical.
With free form lenses you are able to control the powers based on center of rotation, rx, material, etc. Each lens is made independently of the next so there are no molds or Rx ranges to worry about as each degree of eye rotation is calculated and compensated for.
At Zeiss we use Martin's rule to lower the PRP 4mm based on an average tilt of 9 degrees. This allow light to pass through the eyes center of rotation at the point is is supposed and increases to ability to see better.

[Hayde]

What a post, Brent! Thank you. This makes me wonder what brand of aspheric is being referenced in the OP? (...an OP I completely misread when I first posted, apologies kisselt!)

[Kujiradesu]

They're synonyms.

Prism Reference Point is the modern term for Major Reference Point. From the Germans circa 1990's.

Prismenbezugspunkt.

Best regards,

Robert Martellaro

I had a professor who pronounced it: MeRP (like burp), and for some reason this all ways makes me laugh when referring to the MRP.

[Pete Hanlin]

Sorry for resurrecting an old thread. Doubly sorry for missing it in the first place- someone was even nice enough to send me a PM to draw my attention, but I've been somewhat insanely busy at work this month (we're getting ready to launch a number of new products, which creates a lot of work for the Technical Marketing team).

It's moments like this (and there are many such moments) that I miss my friend Darryl Meister. I'd love to see what he would have contributed to this thread (I would have had to read it five times to completely comprehend it, but... :^).

Still, the comments made demonstrate the wealth of knowledge on Optiboard. I'm not going to add much, but I will say one of my favorite products when I was dispensing was by a company named Optima. They had a FSV dual aspheric that was just amazing, IMO. As for digital vs. molded aspherics, I would note that molded aspherics have always been good for plus lenses (because the front surface is where gains can be made on these lenses). However, for minus lenses (I used to be -4.75 prior to LASIK 16 years ago) the main curvature is on the back- so front side asphericity had minimal impact. Minus powered lenses are where digitally surfaced aspherics really shine, IMO- because the asphericity can be applied to the ocular surface. As lensgrinder pointed out, the other advantage of back side aspherics is the ability to take cylinder into account (I think Zeiss actually coined the term "atoricity" to describe a lens that has asphericity applied to both sphere and cylindrical powers).

To the OP (if they're still checking in to this thread), I did want to make one point (especially since it also touches on compensated power). It's a bit incorrect to measure the lens at peripheral points with a lensometer and believe the power of the lens is decreasing in the periphery. To the lensometer, yes- the lens is decreasing in power when you measure through a peripheral point. However, this is not how the eye perceives the power of the lens. Assuming the lens has been properly placed in front of the eye (and yes, as already noted, aspherics should be fitted with extra care to ensure the aspheric feature is centered around the optical axis of the wearer), the eye is going to encounter the periphery of the lens differently than the way the lensometer does. Spherical error occurs when the peripheral rays are encountered by the eye in the as worn position (as the OP noted)- but the lensometer doesn't see spherical error, because the lens as mounted in the lensometer in the periphery will tilt the lens to make it perpendicular to the lens stop, and the lensometer only measures a small portion of the lens at any one time. Lensometers don't do a particularly great job at measuring the "as worn" effect of an Rx at any point other than the optical center (this is why we have compensated powers, because we never measure a PAL's power at it's OC).

As to why the OP may be noting issues with the aspheric lens, again- assuming it was fit properly- it could be you are one of the small percentage of individuals who do not "tolerate" asphericity well. Some research suggests there is a small % of people whose visual systems just do not like asphericity. Why this is so is still a mystery. Considering most modern progressives have asphericity built into the distance, this creates an interesting point to ponder...

Regarding "thinness" vs. "improved optics" it was really nice to read posts by those who have obviously been around for awhile. In the early days of asphericity, the focus (pun intended) was more on restoring optics by reducing spherical distortion (and yes, a good aspheric design should decrease the perception of curvature of field). Very quickly (at least IMO), the direction of aspherics seemed to be "How flat can we make the lens without screwing up the optics?" For some aspheric lenses, the main function of the asphericity seems to be flattening the lens cosmetically while maintaining (but not necessarily improving) the optical properties of a steeper lens. Personally, I think this has gotten us to a point where some lenses are TOO flat to mount in certain frames.

Not sure I added anything, but when I logged on I noted the PM pointing out this thread and found it to be really interesting. In the future, if anyone feels I could add to a discussion (e.g., if there's a question related to an Essilor product), just send a link to the thread to my email address: phanlin@essilorusa.com