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Thread: Lens thickness & fitted back vertex distance

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    Master OptiBoarder OptiBoard Silver Supporter ak47's Avatar
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    Lens thickness & fitted back vertex distance

    I do not understand why we don't have to consider Rx lens thickness when measuring fitted back vertex distance. Especially in a high plus lens the thickness is significant and the "back" of the lens is much closer to the cornea then the demo lens.

    Can someone smarter than me please explain?

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    I've thought this myself. Glazing lenses into a frame can also affect the wrap and panto. Therefore, on high prescriptions and in certain frames, how accurate are we really being when providing POW measurements?

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    What's up? drk's Avatar
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    Are you saying, AK, that we use some vertex-o-meter device, but we're measuring a demo lens, vs. a flat-backed high plus or curvy-backed high minus? I've noted that, too.

    Heck, I've noticed how sloppy vertexometry is, in general, from phoroptor to vertexometer to adjustment to schnozz-skiing.

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    Master OptiBoarder AngeHamm's Avatar
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    Where is the effective "front" of the vertex distance? It seems like it would be the front of the lens, the back of the lens, or the center of the lens. If it's one of the latter two, high hyperopes would certainly be affected. I need some lens guys to chime in here.
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    Master OptiBoarder AngeHamm's Avatar
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    Quote Originally Posted by drk View Post
    Are you saying, AK, that we use some vertex-o-meter device, but we're measuring a demo lens, vs. a flat-backed high plus or curvy-backed high minus? I've noted that, too.
    The high minuses shouldn't really make a difference, as their center thicknesses will be similar.
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    One eye sees, the other feels. OptiBoard Gold Supporter
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    Quote Originally Posted by ak47 View Post
    I do not understand why we don't have to consider Rx lens thickness when measuring fitted back vertex distance. Especially in a high plus lens the thickness is significant and the "back" of the lens is much closer to the cornea then the demo lens.

    Can someone smarter than me please explain?
    The plate height (and sometimes bevel placement) has the greatest influence on the vertex distance (see image below). I see this with iseikonic lenses when an increase in the base curve on a minus lens (to decrease minification) is usually offset by a corresponding increase in the vertex distance (increasing minification). Exceptions are moderate plus full back surface progressive lenses and high plus lenses of all types, where the plate height and back curve approaches zero.

    Quote Originally Posted by AngeHamm View Post
    Where is the effective "front" of the vertex distance? It seems like it would be the front of the lens, the back of the lens, or the center of the lens. If it's one of the latter two, high hyperopes would certainly be affected. I need some lens guys to chime in here.
    Because the effective power of an ophthalmic lens is the back vertex power, the vertex distance is measured from the cornea to the back of the spectacle lens.

    Hope this helps,

    Robert Martellaro
    Attached Thumbnails Attached Thumbnails Plate Height01.gif  
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

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    What's up? drk's Avatar
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    Quote Originally Posted by AngeHamm View Post
    The high minuses shouldn't really make a difference, as their center thicknesses will be similar.
    Ah so

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    Master OptiBoarder AngeHamm's Avatar
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    I knew Robert would come through.
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    Master OptiBoarder OptiBoard Silver Supporter ak47's Avatar
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    But, my question is why are we measuring to the eyewire and not accounting for the thickness of the spectacle lens?

    Quote Originally Posted by Robert Martellaro View Post

    Because the effective power of an ophthalmic lens is the back vertex power, the vertex distance is measured from the cornea to the back of the spectacle lens.

    Hope this helps,

    Robert Martellaro

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    Master OptiBoarder optical24/7's Avatar
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    Quote Originally Posted by ak47 View Post
    I do not understand why we don't have to consider Rx lens thickness when measuring fitted back vertex distance. Especially in a high plus lens the thickness is significant and the "back" of the lens is much closer to the cornea then the demo lens.

    Can someone smarter than me please explain?
    Quote Originally Posted by Robert Martellaro View Post

    Because the effective power of an ophthalmic lens is the back vertex power, the vertex distance is measured from the cornea to the back of the spectacle lens.

    Hope this helps,

    Robert Martellaro
    As Robert stated, the back vertex is where the power of a lens is measured. AK, if you are referring to lens thickness needing to be accounted for, it's compensated at the manufacturer/surfacing lab so the final power is correct at the back vertex.

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    Master OptiBoarder OptiBoard Silver Supporter ak47's Avatar
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    Okay...so what I am hearing is that I am not crazy, back vertex really means from the back of the Rx lens, but the lab accounts for the actual lens thickness so we would be "doubling down" as fitters if we include the Rx thickness in the so-called "back" vertex distance we give to the lab?

    Quote Originally Posted by optical24/7 View Post
    As Robert stated, the back vertex is where the power of a lens is measured. AK, if you are referring to lens thickness needing to be accounted for, it's compensated at the manufacturer/surfacing lab so the final power is correct at the back vertex.

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    What's up? drk's Avatar
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    In my mind, the labs determination of BVP has nothing to do with vertex distance. I think that's a red herring.

    I think, ak, that the difference in vertex between a thick plus lens (like Robert's picture above) and the plastic plano lens (or eyewire) is, unfortunately, going to the slop bucket.

    I don't think that's too great a crime, because, believe me, it's cumbersome to refract at a super-precise (like, +/- 1-2 mm) distance.

    Yes, the errors could be additive, but maybe they'll cancel out.

    I do know, though, that heavy plus lenses always droop into a longer position, so a bias to underplussing when prescribing is not a bad thing (I do that by biasing to the minus on non-presbyopic myopes, too). In fact, a lot of docs "cut the plus", so a little slip won't hurt at all.

    Maybe I'm off-base, here.

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    Master OptiBoarder optical24/7's Avatar
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    Quote Originally Posted by ak47 View Post
    Okay...so what I am hearing is that I am not crazy, back vertex really means from the back of the Rx lens, but the lab accounts for the actual lens thickness so we would be "doubling down" as fitters if we include the Rx thickness in the so-called "back" vertex distance we give to the lab?
    Exactly. Back in my old cataract lens surfacing days, we had charts for how much to compensate the back curve to account for the induced magnification of thick plus lenses and any back vertex differentials from refracted at to wear position. Today, surfacing lab software is way more precise.

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    Master OptiBoarder lensgrinder's Avatar
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    Assuming a demo lens with 6.00 D back curve the sag of the back surface will equal 3.63. Let's assume we place a +5.00 D lens in the same frame with a 7.50 D front surface, the back surface sag will equal 1.85. 3.63 - 1.85 = 1.78 mm. As Robert states the bevel will make a difference as well. Regardless 2 mm movement on a +5.00 D lens will not make a difference (~0.05 D).
    The majority of labs are not compensating lenses for fitted vertex distance since 99.9%(possibly 100%)of prescriptions do not have a refracted vertex. The compensations are based on wrap and pantoscoptic angle. The vertex distance is used for other purposes.

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    VD is not just about power compensation. Along with the actual lens tracing, Top flights progressives can really globally optimize the entire lens surface, especially in progressives.

    B

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    That's true, but if you're starting with an incorrect value at the eye point, then that will surely affect peripheral zones as well.

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    What's up? drk's Avatar
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    Translation? See if I'm speaking optics-ese properly.

    Barry:
    "Hey, you think vertex is important in single vision? Try progressives."

    Casual reader:
    "Yeah, no duh, making sure that people get the right power for the distance portion of their progressives matters too...BARRY!"

    Barry:
    "No, I mean, when you do fancy lens design, the designers expect the center of rotation of the eye to be "x" mm away from the lens, and the designers expect the eye to be "y" mm from the lens periphery (and all that crap) in order to make a smooth, swim-free PAL"

    Casual observer:
    "Uh, yeah. I...I knew what you meant, all along. And my wife is Morgan Fairchild."

    Robert S:
    "Yeah, but it all starts with a right proper vertex so the powers are right, too."

    Barry:
    "I think I just said that."

    Robert S:
    "Did not"

    Barry:
    "Did so"

    Robert S.:
    "Liar"

    Barry:
    "I'm rubber, you're glue"

    Robert S.:
    "I can prove it. There's no such thing as a 'Top Flights' brand progressive!"

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    Master OptiBoarder OptiBoard Silver Supporter lensmanmd's Avatar
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    OMG. I’m in stitches.
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    Hahaha

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    One eye sees, the other feels. OptiBoard Gold Supporter
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    Quote Originally Posted by lensgrinder View Post
    The majority of labs are not compensating lenses for fitted vertex distance since 99.9%(possibly 100%)of prescriptions do not have a refracted vertex.
    Definitely 100% in my area. The last time I saw a vertex distance noted on an Rx (last week I dispensed a -17 PAL and a +14 SuperMod on the same day) was from a local MD about 10 years ago, and that was only because the tech, Lou Messina, was an ex-lab manager. At these powers, we have to hope that the client's head was pressed tight against the phoropter.

    Quote Originally Posted by Barry Santini View Post
    VD is not just about power compensation. Along with the actual lens tracing, Top flights progressives can really globally optimize the entire lens surface, especially in progressives.

    B
    That's the beauty of the more capable optimized lens designs- once the basic design is proven to be comfortable and effective through clinical testing, that design will continue to perform in a similar fashion over a wide range of RXs and fitting values.

    Best regards,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

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    Master OptiBoarder AngeHamm's Avatar
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    Quote Originally Posted by drk View Post
    Translation? See if I'm speaking optics-ese properly.

    Barry:
    "Hey, you think vertex is important in single vision? Try progressives."

    Casual reader:
    "Yeah, no duh, making sure that people get the right power for the distance portion of their progressives matters too...BARRY!"

    Barry:
    "No, I mean, when you do fancy lens design, the designers expect the center of rotation of the eye to be "x" mm away from the lens, and the designers expect the eye to be "y" mm from the lens periphery (and all that crap) in order to make a smooth, swim-free PAL"

    Casual observer:
    "Uh, yeah. I...I knew what you meant, all along. And my wife is Morgan Fairchild."

    Robert S:
    "Yeah, but it all starts with a right proper vertex so the powers are right, too."

    Barry:
    "I think I just said that."

    Robert S:
    "Did not"

    Barry:
    "Did so"

    Robert S.:
    "Liar"

    Barry:
    "I'm rubber, you're glue"

    Robert S.:
    "I can prove it. There's no such thing as a 'Top Flights' brand progressive!"
    Chris R:
    "You wouldn't have to worry about any of this if you put all of your patients in FT28s like any sane person would. With a nice tint in place of superfluous AR coating."

    I'm Andrew Hamm and I approve this message.

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    This is the best :)

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    I miss Chris.

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    One eye sees, the other feels. OptiBoard Gold Supporter
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    “In response to poor fitting in the form of tilt or decentration, lenses with aspheric form surfaces were found to have greater off-axis power errors than best-form lenses with spherical surfaces.” -Atchison, D. A. & Tame

    https://www.ncbi.nlm.nih.gov/pubmed/8278198


    https://www.opticianonline.net/news/...lar-refraction

    https://www.odob.health.nz/wp-conten...f-D-Wilson.pdf
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

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