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Thread: Some wait, but why? questions about lens tilt and OCs

  1. #1
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    Some wait, but why? questions about lens tilt and OCs

    Look at the pic below. We all know that we want the optical axis to pass through the center of rotation and you do this by lowering the OC 1mm for every 2 degrees yada yada. But why. In this instance, the OA goes through the COR, but the eye is gazing forward. So how is this helpful?

    Also, for anisometropic patients, is it better to keep the OC pure as to avoid vertical imbalance?

    Also also, Im assuming this is why zeiss SV have a 4mm drop. Why dont other lenses do this?

    Also also also, what are the rules on tilt and OCs as they apply to aspherics in general?

    Also*4, should there be any OC moment to compensate for faceform? And if not at all then why?

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    Uggghhh. Pic didn’t paste. Here’s the link https://www.2020mag.com/article/opti...-of-lens-forms

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    What's up? drk's Avatar
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    Those are good questions. I'll start the answering, but I'll need help.

    Here's from the piece: "In addition to the oblique astigmatism that occurs while viewing off-axis objects in the periphery, you can introduce oblique astigmatism by simply tilting a lens, since this also places the line of sight at a significant angle to the optical axis of the lens. This is sometimes referred to as "astigmatism due to lens tilt." The oblique astigmatism induced by lens tilt can be minimized by ensuring that the optical axis of the lens passes through the center of rotation (C) of the eye. We can accomplish this by manipulating the relationship between the pantoscopic tilt—which is lens tilt toward the cheeks—and the height (H) of the wearer's pupil center above the optical center (OC) of the lens according to the following rule-of-thumb:Ensure 1 mm of Optical Center Drop (H) for Every 2 of Pantoscopic Tilt"

    I think that's not untrue, but not "correct" because they have the tail wagging the dog. I believe it's like this: "In order for the eye to look through roughly the same optics on downgaze (reading) as for straight-ahead gaze, the optical axis of the lens must intersect the (approximate) center of rotation of the eye."


    Then "In order to acheive this effect, the optic axis must be dropped towards downgaze, and this is done by tilting the lens down and dropping the optical center 1mm per 2 degrees of tilt."


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    What's up? drk's Avatar
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    For anisometropic patients, the same OPTICAL issues apply, but then it's overlaid with the induced prism on downgaze issue. (Ignore multifocals for now...just SV.)

    So...if you follow the original lens tilt/drop the OC protocol, yes, you are introducing prismatic imbalance on straight ahead gaze, but you're reducing it on downgaze. So it kind of splits the pain. And that seems to work.

    Now, having said that, you COULD put the OC dead center for less prismatic imbalance on straight-ahead gaze and have the patient drop their chin to read. I'm not sure what degree of reading downgaze occurs on a single vision patient, but they can always drop the head. But I've never done that.

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    What's up? drk's Avatar
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    As to Zeiss SV, I think they are presuming 8 degrees of panto, not six. Dang Euroweenies.

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    What's up? drk's Avatar
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    As to aspherics/FFSV, I think they have all that worked out in the mathematics. When we drop optical centers and such stuff, we are ackshually doing lens tilt compensation/ray tracing on our own (not unlike using Darryl's wrap compensator, etc.).

    Modern lens companies scornfully laugh at our puny efforts, and they just need the OC height and POW measurements and them and God do the rest.

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    Quote Originally Posted by Prentice Pro 9000 View Post
    Also*4, should there be any OC moment to compensate for faceform? And if not at all then why?
    Maybe I'm thinking about this wrong, but when I think of faceform wrap I think of it as a horizontal issue. So if anything would need to be changed it would be the PD's and not the OC. And if you are using a wrap lens design I believe that is already calculated for by the lab software.

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    What's up? drk's Avatar
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    I see where you're going with the wrap frame..."Hey, if we are trying to keep optics stable as the eyes roll down to read in the vertical dimension, what about keeping the optics stable when the eyes roll left, or right in the horizontal dimension?"

    [Firstly, in the vertical dimension, we are sacrificing upgaze for downgaze. Robbing Peter to pay Paul. But unless Peter is Minnesota Fats (like Robert Martellaro's friends) no one uses upgaze so no big loss.]

    We can't just, say, move the OCs "left" to help left gaze or "right" to help right gaze because it would kill the other direction. PATIENT: "Hey, genius! I love it when I have perfect optics when checking my blind spot...but I can't give the side-glance to my wife without her face looking blurry". ME: "That's a feature, not a bug."

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    What's up? drk's Avatar
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    Quote Originally Posted by NAICITPO View Post
    Maybe I'm thinking about this wrong, but when I think of faceform wrap I think of it as a horizontal issue. So if anything would need to be changed it would be the PD's and not the OC. And if you are using a wrap lens design I believe that is already calculated for by the lab software.
    Indeed. The p.d. is outset for wrap but I think that's more of a geometric distance compensation than a directly optical one. But they do morph the powers to optically optimize clear vision as peripherally as possible.

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    Quote Originally Posted by drk View Post
    Those are good questions. I'll start the answering, but I'll need help.

    Here's from the piece: "In addition to the oblique astigmatism that occurs while viewing off-axis objects in the periphery, you can introduce oblique astigmatism by simply tilting a lens, since this also places the line of sight at a significant angle to the optical axis of the lens. This is sometimes referred to as "astigmatism due to lens tilt." The oblique astigmatism induced by lens tilt can be minimized by ensuring that the optical axis of the lens passes through the center of rotation (C) of the eye. We can accomplish this by manipulating the relationship between the pantoscopic tiltwhich is lens tilt toward the cheeksand the height (H) of the wearer's pupil center above the optical center (OC) of the lens according to the following rule-of-thumb:Ensure 1 mm of Optical Center Drop (H) for Every 2 of Pantoscopic Tilt"

    I think that's not untrue, but not "correct" because they have the tail wagging the dog. I believe it's like this: "In order for the eye to look through roughly the same optics on downgaze (reading) as for straight-ahead gaze, the optical axis of the lens must intersect the (approximate) center of rotation of the eye."


    Then "In order to acheive this effect, the optic axis must be dropped towards downgaze, and this is done by tilting the lens down and dropping the optical center 1mm per 2 degrees of tilt."

    I think my issue is that this 2degree/1mm ratio tule is mentioned but then its never drawn out as to why or how it actually improves the straight ahead vision. For as detailed as that article is, its never really laid. And your point is taken, but isnt the Downgaze really reeeeaally a secondary consideration to straight ahead gaze?

  11. #11
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    Quote Originally Posted by drk View Post
    As to Zeiss SV, I think they are presuming 8 degrees of panto, not six. Dang Euroweenies.
    for the zeiss indv SV there’s a 4mm drop.

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    Quote Originally Posted by NAICITPO View Post
    Maybe I'm thinking about this wrong, but when I think of faceform wrap I think of it as a horizontal issue. So if anything would need to be changed it would be the PD's and not the OC. And if you are using a wrap lens design I believe that is already calculated for by the lab software.
    no for sure, I meant the “OC 1mm” rule being applied to PDs

  13. #13
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    Quote Originally Posted by drk View Post
    I see where you're going with the wrap frame..."Hey, if we are trying to keep optics stable as the eyes roll down to read in the vertical dimension, what about keeping the optics stable when the eyes roll left, or right in the horizontal dimension?"

    [Firstly, in the vertical dimension, we are sacrificing upgaze for downgaze. Robbing Peter to pay Paul. But unless Peter is Minnesota Fats (like Robert Martellaro's friends) no one uses upgaze so no big loss.]

    We can't just, say, move the OCs "left" to help left gaze or "right" to help right gaze because it would kill the other direction. PATIENT: "Hey, genius! I love it when I have perfect optics when checking my blind spot...but I can't give the side-glance to my wife without her face looking blurry". ME: "That's a feature, not a bug."
    ah yes, better for one eye maybe, but not for both. So it an understood compromise that the horizontal plane of vision may not pass through the COR?

  14. #14
    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    It’s easiest to think of the lens realignment obtained through pantoscopic angle as reducing variations in VD as the eye make gaze angle excursions. Using martin’s rule helps to balance off axis aberrations, but cannot eliminate them.

    If the lowering of the OC in Martin’s rule followed an arc rather than a linear drop, there would be even less optical errors on the straight ahead gaze angle when using a best form CC lens. But in as much as the straight ahead gaze intersects the lens at an angle when Panto and OC are not in the expected orthodox relationship, the net result is optical error and the need for compensating the Rx at this optical area.

    B

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    Quote Originally Posted by Barry Santini View Post
    Its easiest to think of the lens realignment obtained through pantoscopic angle as reducing variations in VD as the eye make gaze angle excursions. Using martins rule helps to balance off axis aberrations, but cannot eliminate them.

    If the lowering of the OC in Martins rule followed an arc rather than a linear drop, there would be even less optical errors on the straight ahead gaze angle when using a best form CC lens. But in as much as the straight ahead gaze intersects the lens at an angle when Panto and OC are not in the expected orthodox relationship, the net result is optical error and the need for compensating the Rx at this optical area.

    B
    thanks Barry. May I ask, where is this info available to peruse? I tried every way of googling and looked in multiple books and have not seen this explanation. Also, if this 2:1 ratio rule is meant to reduce optical errors, then would we actually fit Freedom SV at the pure OC with the expectation that the lens design would compensate for these errors?

    Thanks in advance

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    This isn't findable in a convenient search.

    I only just learned and better understood this from taking a very high-end, full semester webinar out of Australia in the first half of this year. It required me to rise every Tuesday between 3AM and 5AM every week from late January to late May.
    Well worth the money and time. Honestly, I actually looked forward to it more than getting up early to start vacation!

    It is put on by the Academy for Advanced Ophthalmic Ophthalmic Optics, run by Dr. Thao Hannaford, OD, and Grant Hannaford, Optician, who was voted International Optician of the year in 2022! Grant is currently completing his Phd in Optics. Grant is effectively the new “Darryl.”

    They say if you cant explain a complex topic in a simple manner, you really don’t understand it. Well these two mentors definitely can do that! They were clearly willing to spend whatever time was necessary handling any question you have.

    They are, by their own mission statement, committed to disrupting the high end of the ophthalmic market, instead of what we see mostly happening on the broad optical landscape now—disruptors functioning at the low end of the market.

    https://aaoo.net.au/

    B
    Last edited by Barry Santini; 06-08-2023 at 06:35 AM.

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    Martin’s rule is too simplistic. It is based on model eye averages that, in their most basic fault, fail to take into account the basic changes in eye biometrics that follow the actual Rx.

    And if you’ve experienced post cataract patients who had previously worn progressives comfortably and successfully, but after surgery, struggle to get re-acquainted and find good utility when refitted with progressives after surgery, I can tell you that it’s, in large part, because their progressive design is based on biometrics connected with new, near-emmetropic prescriptions, and not the REAL biometrics of their eye, which was their life-long ammetropia.

    The next big advanced in ophthalmic optics will be the integration of an individual eye’s biometrics into optimizing a larger area of binocular summation through enhanced retinal engagement, which results in the as worn glasses yielding better acuity than found during the exam.

    Believe it, or not.

    I can tell you this: The era of learning the full scope of opticianry “on the job” or through an apprentice track, is essentially over.

    RIP being the best optician without formal schooling.
    AND I’M NOT TALKING LICENSURE!
    I am talking formal classroom exposure.

    Stop rallying around protecting your license.

    And start rallying around becoming the best optician you can.
    If you do, the Warby’s, Online’s and other low end disrupters do not have any chance of attracting the clients you should be seeking—The ones willing to pay for, the best products and service possible.

    B
    Last edited by Barry Santini; 06-08-2023 at 06:36 AM.

  18. #18
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    Quote Originally Posted by Barry Santini View Post
    This isn't findable in a convenient search.

    I only just learned and better understood this from taking a very high-end, full semester webinar out of Australia in the first half of this year. It required me to rise every Tuesday between 3AM and 5AM every week from late January to late March.
    Well worth the money and time. Honestly, I actually looked forward to it more than getting up early to start vacation!

    It is put on by the Academy for Advanced Ophthalmic Ophthalmic Optics, run by Dr. Thao Hannaford, OD, and Grant Hannaford, Optician, who was voted International Optician of the year in 2022! Grant is currently completing his Phd in Optics. Grant is effectively the new Darryl.

    They say if you cant explain a complex topic in a simple manner, you really dont understand it. Well these two mentors definitely can do that! They were clearly willing to spend whatever time was necessary handling any question you have.

    They are, by their own mission statement, committed to disrupting the high end of the ophthalmic market, instead of what we see mostly happening on the broad optical landscape nowdisruptors functioning at the low end of the market.

    https://aaoo.net.au/

    B
    wowzers!!! Thank you for this!

  19. #19
    What's up? drk's Avatar
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    They say if you cant explain a complex topic in a simple manner, you really don’t understand it.
    Truth.

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    Quote Originally Posted by Barry Santini View Post
    Martin’s rule is too simplistic. It is based on model eye averages that, in their most basic fault, fail to take into account the basic changes in eye biometrics that follow the actual Rx.

    And if you’ve experienced post cataract patients who had previously worn progressives comfortably and successfully, but after surgery, struggle to get re-acquainted and find good utility when refitted with progressives after surgery, I can tell you that it’s, in large part, because their progressive design is based on biometrics connected with new, near-emmetropic prescriptions, and not the REAL biometrics of their eye, which was their life-long ammetropia.

    The next big advanced in ophthalmic optics will be the integration of an individual eye’s biometrics into optimizing a larger area of retinal engagement, which results in the as worn glasses yielding better acuity than found during the exam.

    Believe it, or not.

    I can tell you this: The era of learning the full scope of opticianry “on the job” or through an apprentice track, is essentially over.

    RIP being the best optician without formal schooling.
    AND I’M NOT TALKING LICENSURE!
    I am talking formal classroom exposure.

    Stop rallying around protecting your license.

    And start rallying around becoming the best optician you can.
    If you do, the Warby’s, Online’s and other low end disrupters do not have any chance of attracting the clients you should be seeking—The ones willing to pay for, the best products and service possible.

    B
    Other than the Australian link, are there other resources you would suggest? Heck are there entire courses out there?

  21. #21
    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    I Bekieve there is also a Course path from Advanced Ophthalmic Systems in the UK.

    Barry

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    Hi Prentice Pro 9000.

    Go to http://216.144.236.77/tools/spectacle_optics.php

    Open program (Windows only, and you might need an older version- I use XP with a virtual machine on macOS) and select "optical analysis". Enter the parameters, run. The picture below shows an example where the optician selected too flat of a base curve for the power needed, resulting in a high degree of off-axis aberrations (marginal astigmatism and power error). Use the asphericity slider or steepen the BC to reduce these errors.

    Mispositioning the vertical OCs will introduce aberrations both on and off-axis for all types of lenses, and will be significant on medium and high dioptric powers, especially those with aspheric/atoric surface designs.

    If you need to match the vertical OC with the pupil heights to reduce on-axis vertical prism imbalance (VI), use a lens that has the vertical OC at the pupil height, where the software engine can correct for these aberrations. For PALs, you will probably need to use a design with no drop, or use canceling prism in one eye, aligning the pupil height with the prism reference point (PRP). Keep in mind that this will increase VI on the downgaze- sometimes it best to compromise, equalizing the VI somewhat at both angles of gaze, or use slabbed segmented multifocal, SVDO, etc.

    Hope this helps,

    Robert Martellaro
    Attached Thumbnails Attached Thumbnails Optical Analysis.jpg  
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    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.



  23. #23
    What's up? drk's Avatar
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    Quote Originally Posted by Prentice Pro 9000 View Post
    I think my issue is that this 2degree/1mm ratio tule is mentioned but then it’s never drawn out as to why or how it actually improves the straight ahead vision. For as detailed as that article is, it’s never really laid. And your point is taken, but isn’t the Downgaze really reeeeaally a secondary consideration to straight ahead gaze?
    I'm pretty sure that we are sacrificing distance optics a tad, in order to help downgaze reading (and again, this is SV!).

  24. #24
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    Quote Originally Posted by drk View Post
    I'm pretty sure that we are sacrificing distance optics a tad, in order to help downgaze reading (and again, this is SV!).
    Not exactly. Aligning the optical axis of the lens with the center of rotation of the eye minimizes aberrations at all all angles of gaze. Tilting the lens around the horizontal axis (panto/retro) also increases the sphere power and introduces cylinder with the sign equal to the sphere power, although the errors are small enough to be safely ignored with low to medium powers and low tilt values.

    Remember when we were myopic kids and we would tilt the lenses at a crazy steep angle to see the board clearer? We knew it was time for another bump in power- off we went to the refractionist.

    Robert
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    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.



  25. #25
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    Barry-what are you referring to as an eye's "biometrics"?

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