Double Aspheric Progressives

[AndyOptom]

It's the season, apparently, so the lens reps have been coming by more often. Including that elusive specimen, the Tokai lens rep! Poor gent is currently single handedly servicing something like over a hundred accounts, I think, so I'm glad to see him at all. But, I digress.

So the Tokai rep is singing praises of his lens designs, and for the first time since... 2016 (?), I find myself being told about double aspheric progressives. As in, back surface uses aspheric and/or atoric optics to optimise the Rx and so on, but the front surface is also custom ground to be a rotationally symmetric aspheric surface.

After giving it some thought, I realised I never did find the answer to that question which nagged at me years ago:

If a double aspheric back surface progressive uses a rotationally symmetric aspheric front surface... this means the asphericity etc of the front would be with reference to a single point on the front surface. The back surface of the lens is the corridor and progressive optics, so that whole back surface lacks rotational symmetry or even an optical center anyway. So, logically, would double aspheric progressives grind the rotationally symmetric aspheric front surface, centered around the prism reference point since that's usually the geometric center of the blank also?

Frankly, I don't doubt that a major Japanese lens lab like Tokai would actually know what they're talking about (and doing to their products), and I suspect the ambiguity in explanation or lack of explanations outright is simply due to language barriers and lack of translation resources. But I do wonder about this particular feature, since if the front surface asphericity is centered at the PRP, wouldn't that compromise the far AND near vision zones somewhat, compared to a spherical front? After all, we all know the effects of decentering aspheric lenses is hardly among our preferred optical outcomes.

Or, if the asphericity is so gradual as to only start being significant beyond the MRP and near circle itself, wouldn't most of this aspheric zone be cut off anyway, making such features a moot point?

[drk]

AO: let's lay the groundwork so I can follow.

1. Most designs right now use a spherical front surface and do the stuff on the back side.
2. Tokai is doing an aspheric (degree unknown, but non-"toroidal") front surface and stuff on the back side.

Right?

If so, can you understand why they would try to do this? What is the potential advantage? Lens mass?

[AndyOptom]

Hello hello doc.

1. Indeed, that is my understanding as well. Most modern PAL rely on spherical front single vision blanks + free form grinding.

2. For selected designs only, but yes. Pretty much making the front surface aspheric but equally so in all directions, hence rotationally symmetric aspheric. Unlike Hoya's double surface designs, which are technically aspheric on both surfaces... but in the form of cross cylinders instead.

As to why Tokai allegedly does this, the benefits quoted to me were as follows, and to be fair, I do see some (albeit limited) logic to some of it.

- Thickness/mass control, even after accounting for index and frame selection. Logical, since the correlation between aspherics and cosmetics is tried and tested.
- Less swim. Somewhat logical, though I think we all know this comes at the cost of other types of vision compromise. IIRC applicable in the case of Varilux S and X.
- Compensation for the excessively curved blanks necessary for a full back surface progressive. Ditto for Varilux S and X.

I don't doubt the logic (however slight) of these claims. However, I'm wondering about the potential trade off, since I'm a firm believer in dispensing optics being the art and science of compromise... and if anyone cites me an area of improvement in optics for a given product, my first question is always, "What's the catch?".


Frankly, even I'd admit that I'm splitting hairs here, the lenses would likely work well enough anyway. But, why not ask and potentially learn something, I do love my lens concepts... and if it somehow, someday proves useful to anyone, the time and effort of enquiry was well spent IMHO :)

[drk]

I'm not trying to sound smart on this, because I ain't!

I would imagine that flattening/mass would be the main advantage.
I would imagine that you could do only a marginal amount (I don't know how we specify asphericity--used to be "drop", now I think it's "eccentricity value").
I would imagine that there is only the ability to clean up the distance portion slightly allowing some play with maximizing the bottom of the lens.

That's all I can imagine.

[AndyOptom]

Update! Since it's been a busy couple of weeks, and I'm finally catching up on the online world...

Agreed with what you mentioned on the potential perceived improvements.

In the meantime since I last logged in, I did fit and dispense a pair of Tokai progressives to a patient who was a low hyperope, first time wearer. Not the double aspheric design, but she was quite happy with the end result (clarity, comfort, and thin lenses). Admittedly, not much of a WOW! factor relative to what I'd expect from Nikon or Hoya, considering local Tokai pricing.

If someday I do get a chance to dispense the double aspheric version, will update with the result.

[drk]

Keep posting!

[AndyOptom]

Update:

Ordered a pair of the entry level Tokai progressives in 1.60 for about -6ish in both eyes, Add 1.50. Metal full frame, fitting was within what I'd consider good tolerances, and I requested a 40/60 bevel just in case. Took a couple of weeks to get done, including edging by the local lab.

As I was away the few days when they got in and the patient collected the finished glasses, just have a colleague's feedback that the lenses were cosmetically good, maybe a couple millimeters sticking out the back on top of the frame at most.

Patient response was allegedly good, and compared to their previous pair of unknown progressives with almost equal distance Rx but lower Add, all felt well with the exception of slightly narrower distance (resolved by adjusting the nose pads to slightly reduce vertex distance).

Will be trying the Tokai designs more if suitable patients present, meanwhile, looks like the performance of their double aspheric designs will remain mysterious to me 😅

[Uncle Fester]

I don't doubt the logic (however slight) of these claims. However, I'm wondering about the potential trade off, since I'm a firm believer in dispensing optics being the art and science of compromise... and if anyone cites me an area of improvement in optics for a given product, my first question is always, "What's the catch?".

If like me you are reading this thread and feel a little overwhelmed (as in smoke coming from your ears starts to fill the room) check out this link to see why a "Perfect" progressive with no compromises in some part of the lens is not possible but run a fan to the outside to vent the room :

From the Hall of Fame where all the threads are delicious!!!

Seidel Aberrations-

http://www.quadibloc.com/science/opt0505.htm

[DanLiv]

I completely agree with your logical skepticism of the concept AndyOptom.

The limited area of improved optics at the aspheric "button" would somehow have to be widened to encapsulate both the DV and NV portions of the progressive design. Modern long corridor digital designs in modern large B frames are going to have a distance of as much as 20mm from fitting height to bottom of NV area, and you're going to need another 5mm above the fitting height for DV. If you make the spherical button at least 25mm diameter to capture all that viewing area, that's going to traverse the entire usable visual area of the progressive. Any asphericity that may be left at the extreme edges would be purely for marginal thinning effect, well outside of visual areas.

A rotationally symmetric aspheric front surface is only going to be optically complementary to a rotationally symmetric back surface. We already know an aspheric front surface technically fails for even for even a simple SV toric Rx. That's what atoric is for. We also already know of advanced dual sided progressive designs, namely front-molded designs like Varilux and Hoya, and the variable curvature of Camber. None of those are rotationally symmetric, nor should they be because progressives are not rotationally symmetric.

I don't see any advantage to this design other than compromising optics to exploit aspheric thinning. If there is something more to the Tokai designs, the rep doesn't understand it enough to explain it either.

[Panlees]

其实，双非球面透镜就是在非球面技术的基础上升级而来的一种透镜。通过使镜片的外表面与内表面完美贴合，通过叠加两个表面来抵消 镜片的厚度，有效减少因边缘曲率引起的视觉失真。因此，成像更清晰、更逼真，实现了全景宽视野。

[drk]

通过使镜 片的外表面 与内表面 完美贴合

[DanLiv]

If Google Translate is to be trusted, this is simply a restating of the claim of optical improvement without any explanation how, which is exactly what OP got from his rep. If you can't explain it to me simply, then you don't understand it well enough.

[AndyOptom]

I completely agree with your logical skepticism of the concept AndyOptom.

The limited area of improved optics at the aspheric "button" would somehow have to be widened to encapsulate both the DV and NV portions of the progressive design. Modern long corridor digital designs in modern large B frames are going to have a distance of as much as 20mm from fitting height to bottom of NV area, and you're going to need another 5mm above the fitting height for DV. If you make the spherical button at least 25mm diameter to capture all that viewing area, that's going to traverse the entire usable visual area of the progressive. Any asphericity that may be left at the extreme edges would be purely for marginal thinning effect, well outside of visual areas.

A rotationally symmetric aspheric front surface is only going to be optically complementary to a rotationally symmetric back surface. We already know an aspheric front surface technically fails for even for even a simple SV toric Rx. That's what atoric is for. We also already know of advanced dual sided progressive designs, namely front-molded designs like Varilux and Hoya, and the variable curvature of Camber. None of those are rotationally symmetric, nor should they be because progressives are not rotationally symmetric.

I don't see any advantage to this design other than compromising optics to exploit aspheric thinning. If there is something more to the Tokai designs, the rep doesn't understand it enough to explain it either.

My thoughts exactly.

But, I'll admit to being enough of a dispensing gambler... I might yet try a pair of these on a patient whose fitting warrants a lens that's as flat as possible. Maybe based on the feedback etc, there might be some clues as to how these things function.

Will see if they match up to performance as I'd expect from Hoya and Nikon designs, which usually tend to be my picks for flatter fittings.

[Prentice Pro 9000]

What I don't get is this:

Front side is aspheric... okay I understand that.
Backside has all the progressive features... so how can you really call that "aspheric". Atoric in parts, yes. Definetly "not spherical", sure. But I don't see the benefit of calling it "aspheric"... like... my iphone is aspheric by that defintion.

[AndyOptom]

As it was explained to me (I forget who and when, it was over a decade ago), some progressive designs flatten the edges of the lens (similar to back surface lenticulation) to get a thinner edge profile. Which is itself a kind of asphericity, distinct from the aspheric change along the corridor.

It's a known property of some designs in the Hoya and Nikon families (and has been referred to using the term 'aspheric' by at least one of them in official marketing material), wouldn't be surprised if Tokai does similar.