Are you speaking of pantoscopic angle bringing the add power closer to the eye here?
Are you speaking of pantoscopic angle bringing the add power closer to the eye here?
Sorry, bad at computers.
Because the backside is produced on a free-form generator, they can modify the progressive optics as needed, with the appropriate software of course. The Physio Short for example, in Trivex, starts out with a regular semifinished Physio blank. The manufacturing complexity is higher, but inventory cost is lower.
This could then technically be called a free-form lens, although it's probably lacking any additional optimizations compared to what could have been an off-the-shelve semifinished PAL. But the Physio Enhanced is absolutely and without any doubt a free-form lens with optimizations.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
No, the umbilic of a "front side" progressive must always offer increasing plus powers, so from the distance reference point it must turn "in" closer to the wearer. The add of "front-side" add lenses will always sit behind, not in front of, the distance spherical power. If it stuck in front even for a tiny bit it would render some portion of the corridor unusable, it would be over minused.
I will disagree with Younger here, the "backside add" (really backside transitional curves) is extremely valuable because there is enormously more power variation outside of the corrective curves when the transition curves are on the front, vs. the back.
Its not unlike Edward Tillyers work in the 1920's to prove Cylinder correction should be on the back, not the front of a lens. Sadly most of what his work wasn't fully appreciated and implemented until the early 1970's.
Since the front side add sits behind the spherical curve, they never intersect on any plane. To smoothly merge those curves ( a requirement of a progressive being "no-line") I will need two transitional curves to do so, one with high plus cyl and one in the reverse direction with high minus cyl. Its sort of an S shape from a cross section. When the transitional curves are on the back, the add is flattest portion of the lens and sphere power is always steeper. The result is the Add power always intersects the plane of the sphere. I only need one transitional curve.
the net result is that although both create distortion, the total number of focal points will be spread over a wider area than a single unwanted cylinder curve would be. It will also require more "space" to resolve both curve smoothly. Less range in focal points, equals less swim or sway. Less space equals greater (wider) usable zones. It creates its own base curve issues, so it necessarily doesn't cut the distortion in half (100% backside free-form lenses don't use Vogels rule for base curves, esp in plus powers).
We always focus on the corrective curves, but the non-corrective curves have a huge influence over the lens.
Here is the way to see it more simply.
Here is the curve line of Traditional front lens:
Convex curve for Distance,
Steeper Convex curve for first transition curve (beginning of soft focus area, very high plus cyl
Steep Concave Curve to link the first Convex curve to the reading. (end soft focus area)
Steep Convex curve for the reading zone (usable)
Steep Concave Curve (unusable)
Steep Convex Curve (unusable)
Convex Curve (usable distance portion)
Backside 100% Free-form lens
Steep Concave Curve (for distance)
Flatter Concave curve (to link distance to reading, soft focus area, minus cyl only)
Flat Concave curve (usable reading area)
Flatter Concave curve (soft focus area, minus cyl only)
Steep concave curve (for usable distance zone)
The backside Free-form lens is ALL Concave surfaces, there is patent that ensure this for most lenses.
Mixing Convex and Concave surfaces on the front of a progressive (As in all traditional and hybrid lenses) creates far great variation in unwanted cyl and prism. In a backside add lens, all the surfaces are concave, of varing degrees. There is distortion, but the variance in unwanted focal points is far less. Less swim or sway. Voila.
Me thinks that Younger knows this, its what makes their newer Camber lens so intriguing, it does not have any concave surfaces on the front, its all convex. Would Younger care to comment?
The simplest way to visual is this, is that the 2 front transitional curves on "front side add" progressive will pull the path of light in 2 directions off center. Combined they created their own unwanted prism that is also divergent from the visual axis. In a backside add lens, that transitional curve only pulls the light path in 1 direction. Both create distortion, but they vary by the type, direction and degree.
There will generally be more swim or sway in a front side add lens as that light is pulled off to a higher net variance between the plus and minus cyl. The math is interesting on this.
A single-vision aspheric blank is rotationally symmetrical. A plus-cylinder lens (for example) would not be.
There's no such thing as a "free-world patent." The '713 patent is a U.S. patent; there's a similar one in Australia, and a somewhat less-broad variant in Europe.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
I read your original comment thus: "Why would Zeiss switch to aspheric blanks when they have a patent on backside progressives using rotationally-symmetrical spherical ones," implying that backside progressives using aspheric blanks would be excluded from their patent because they're not rotationally symmetrical.
Sorry if I read that incorrectly, but that still looks like a reasonable interpretation to me.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
I had a piece of marketing material that implied they were using the original GT2 semi-finished front add blanks. If that is true, then moving more of those blanks would provide some incentive. When I finally got around to ordering the lens for myself the marketing had changed about 6 months later. So I can't verify that the lens was ever a hybrid (full cast add on the front, digital back) or if the marketing material was simply wrong. It was just implied.
Sadly our source as Zeiss has passed away...
The press releases from Germany regarding the GT2-3D launch never say its 100% backside Free-form. The press releases for the GT2-3D-V however, do make that clear. The GT2-3D-V touts in being 100% backside free-form. It may be a marketing error, but there is no mention of Free-form in the orginal GT2-3D release.
http://www.meditec.zeiss.com/C1256A7...2573CC004E9B6D
Dude. This thread has become fantastic.
I have another question about Choice Plus. Is it based on anything? Like the GT2-3D is, presumably, a GT2 design converted to be used on freeform generators with single vision lens blanks, but otherwise pretty similar to a GT2 (except less swim because the power is ground into the back of the lens?). Otherwise, why call it a GT2-3D? If it is mostly new design based on the GT, it should be a GT3. :P. So now transfer that line of reasoning over to the Choice line of lenses. Is Choice Plus just an Individual2 without customization features, with the option for set corridors? Or is it just a new version of a previous Zeiss design? Or is it something completely different than either of those options?
If that ramble makes it sound like I'm confused, it's because I am. The marketing material is most unhelpful in distinguishing WTF these lenses are, and on which patients they should be used. They basically just "good better best" them, but I can't believe that's how it really is. When should I use an Individual over a Choice Plus? Or vise versa?
And, speaking of Individuals, are digital (or freeform, if there is a difference in this case?) single vision lenses any better than conventional? I got a pair of Essilor 360 lenses, and beyond really liking the newest formulation of the Crizal Forte UV coating (they really fixed a lot of my crazing and glare issues last time they revamped it), I notice no difference. I'm a -8.00 -1.00 in my bad eye. I'd think if any myop should be noticing a difference, I should be. Should I try a Zeiss Individual single vision lens next time I get the chance, or will it make no real difference over a stock lens?
Thanks. Hopefully those questions made sense.
Last edited by stedel; 10-11-2014 at 09:31 PM.
http://www.optiboard.com/forums/show...l=1#post456317
The answer is complicated. Individual sensitivities vary significantly for one thing.Should I try a Zeiss Individual single vision lens next time I get the chance, or will it make no real difference over a stock lens?
At this power, a premium stock SV finished lens can perform very well and may be hard to beat. Semifinished aspheric lenses, especially in the higher refractive indexes, may suffer from a lack of base curves compared to spherical blanks. We won't use a spherical blank with a power of -8.50 because the best form BC of 1.74 is a rather steep +4.25, increasing thickness, weight, and sag depth, potentially introducing noticeable aberrations off-axis, primarily oblique astigmatism. If the software compensates for tilt, there can be improvement in vision on-axis as well as off-axis, especially when the tilt values are greater than about five degrees.
For example, with your Rx (axis 180) the client chooses a frame that results in a pantoscopic tilt of 12 degrees, and can not be reduced to a lower value due to the endpiece design. Wrap is a typical five degrees. The compensated Rx is about -7.75 -.62 x 16. Without the compensation, the client will be over-minused, with incorrect cylinder and axis values. An emerging presbyope will be most unhappy!
With the cost of optimized free-form lenses decreasing, approaching the cost of semifinished, it makes sense to take advantage of this technology. However, make sure that your clients have been provided with realistic expectations when the expected improvement in optics is small.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Robert, as ususal has a great answer. Small addition: If a SV Lens has prism you are likely to benefit much from Free-form powers. Some stock Aspheric lenses esp Zeiss and Seiko can be really good for Mid powers, and lower cyls.
My best guidelines are that over +2.50 to -4.00 sphere, anything with prism, and any cyl over -1.00 will see the best improvements in SV Free-form. Anything over a +1.50 Cyl should be higher end free-form with POW Compensations (if you can get them).
So far so good with the Auto III, we have successfully re-fit several Physio Enhanced pat's.
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