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Thread: Setting optical center heights for high myopia, index 1.74.

  1. #1
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    Setting optical center heights for high myopia, index 1.74.

    With OC height set to the geo center, there are slightly visible chromatic aberrations when looking straight, the patient tends to look slightly downwards with the head bent upwards to better his vision and it causes discomfort. The geocenter is 5-6mm lower than the pupils, but sweat or regular nose pad deregulation can lower it to 8mm.

    I've read the topic on OC heights: https://www.optiboard.com/forums/sho...238-OC-Heights
    I think it makes complete sense to not set the OC height to the very place the pupils are in, as the nose pads will deregulate a bit with time leading to the OC higher than the pupils which is an even worse situation that the current one. Also, the aberrations would be stronger in the lower lens part, fe. when reading or checking one's phone.

    I'm planning to set them to 4mm above the geocenter, but I'm worried about the amplified lower lens part aberrations, ie. about discomfort when checking one's phone etc.. If there have to be aberrations the patient prefers to have them below the pupils rather than in front of them. Should I go for 3mm as people in the thread I mentioned do or 4mm, what do you think?
    Last edited by donjuan; 09-20-2019 at 11:13 AM.

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    Master OptiBoarder AngeHamm's Avatar
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    What's the RX? What's the material? How are the OCs of previous pairs aligned?
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    Sphere -7.5D, cylinder -0.75D. It's a Chinese lens brand, index 1.74, aspheric design, plastic material (I could try asking the producer for precise compound name they use, but it will take a while and I'm not even sure if I manage to obtain it as their support isn't the best, it's definitely not trivex or anything like that). Previous pair is 8 years old, has very narrow lenses (~25mm lens height) with OC=geocentre, is 2 dioptres weaker and has no cylinders, the patient barely sees anything in them and hasn't used them for a year when he started wearing solely contact lenses. The new glasses have round lens, 42mm high, they fit him perfectly.

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    Master OptiBoarder AngeHamm's Avatar
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    There are a few things going on here. (My more technically experienced colleagues will have much more comprehensive opinions on the subject, though you will probably get more eyes on it in the "General" section than this one.)

    1) I recommend placing the OC much, much closer to the pupil than you're talking about here. The higher the power, the less decentration it takes to really muck things up.

    2) The higher B measurement is also a likely culprit. There is more lens farther off axis, and that portion of the lens will show more chromatic aberration.

    3) 1.74 materials all have pretty low abbe values, so CA will always be an issue. This is where one of the most important things we do is to manage our patients' expectations. With their higher RX, hyper high index lenses are definitely called for, but CA is an unavoidable side effect. Make sure the patient knows it's going to be there before finalizing the sale and your patient will almost always be fine with it.
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    My 2 cents... The old glasses of your patient was spheric or aspheric design? Some people don't "digest" aspheric design.


    aspheric design you have to set fitting heigh on
    the center of rotation of the eye, spot the pupil on glasses with the patient's face at 0 patoscopic tilt... so you will have the right drop oc.

    Eventually can you ask to your lab a 1.74 lens with spheric design.


    I believe 1.74 lenses was all same material: mr-174 abbe 32
    Last edited by dima; 09-20-2019 at 03:38 PM.

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    One eye sees, the other feels. OptiBoard Gold Supporter
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    Quote Originally Posted by dima View Post
    My 2 cents... The old glasses of your patient was spheric or aspheric design? Some people don't "digest" aspheric design.


    aspheric design you have to set fitting heigh on
    the center of rotation of the eye, spot the pupil on glasses with the patient's face at 0 patoscopic tilt... so you will have the right drop oc.

    Eventually can you ask to your lab a 1.74 lens with spheric design.
    Hi Dima.

    That would work, however the best form base curve of a spherical surface design and a high refractive index material for a high myope (in this case about -8.00 D) is probably close to +6 or more, resulting in poor cosmesis. In general, as the refractive index increases, so does the best form BC. And once over 8 D, best form breaks down and asphericity must be employed to reduce oblique astigmatism and power error.

    http://64.50.176.246/cecourse.php?url=lens_design/

    Quote Originally Posted by donjuan View Post
    With OC height set to the geo center, there are slightly visible chromatic aberrations when looking straight, the patient tends to look slightly downwards with the head bent upwards to better his vision and it causes discomfort. The geocenter is 5-6mm lower than the pupils, but sweat or regular nose pad deregulation can lower it to 8mm.
    Yes, try to position the Prism Reference Points as close to the pupil heights as possible, reducing the pantoscopic tilt as needed to compensate, resulting in a compromise between acuity and cosmesis.

    A frame that positions the PRP (180/datum line) closer to the the pupil heights would be ideal.

    Hope this helps,

    Robert Martellaro
    Last edited by Robert Martellaro; 09-23-2019 at 11:35 AM.
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    Master OptiBoarder AngeHamm's Avatar
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    I learned the word "cosmesis" today.
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    One eye sees, the other feels. OptiBoard Gold Supporter
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    Quote Originally Posted by AngeHamm View Post
    I learned the word "cosmesis" today.
    It's the "look good" part of the ophthalmic optics battle between cosmesis and functionality.

    Last edited by Robert Martellaro; 09-23-2019 at 02:35 PM.
    Roberts Optical Ltd.
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    Master OptiBoarder AngeHamm's Avatar
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    Quote Originally Posted by Robert Martellaro View Post
    It's the "look good" part of the ophthalmic optics battle between cosmesis and functionality.
    So I intuited. Excellent word. I'm going to be using it a bunch.
    I'm Andrew Hamm and I approve this message.

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    Quote Originally Posted by AngeHamm View Post
    So I intuited. Excellent word. I'm going to be using it a bunch.
    TBF cosmesis is the cosmetic portion of surgery or prosthetic limbs. I wouldn't say it applies to eyeglasses. I believe cosmetics would be the appropriate term for eyeglasses.

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    One eye sees, the other feels. OptiBoard Gold Supporter
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    Quote Originally Posted by Kwill212 View Post
    TBF cosmesis is the cosmetic portion of surgery or prosthetic limbs. I wouldn't say it applies to eyeglasses. I believe cosmetics would be the appropriate term for eyeglasses.
    I got it from D. Meister. Paste this into you favorite search engine: "cosmesis" spectacle lenses.

    It's a bit of a ten dollar word, but it seems to be common in the ophthalmic community.

    Best regards,

    Robert Martellaro
    Roberts Optical Ltd.
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    www.roberts-optical.com
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    @ Robert Thanks for explanation on aspherics.

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    Quote Originally Posted by Robert Martellaro View Post
    I got it from D. Meister. Paste this into you favorite search engine: "cosmesis" spectacle lenses.

    It's a bit of a ten dollar word, but it seems to be common in the ophthalmic community.

    Best regards,

    Robert Martellaro

    Someone should tell Merriam-Webster then. It seems like cosmesis could be used in reference to the way glasses or lenses make our eyes/faces appear, not in the way the glasses or lenses themselves look.

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    Is that the same Merriam Webster that sits on a bench next to the Ohio River in Rosiclare Illinois and complains about caterpillars on her tomato bushes? I know her! I’ll let her know!

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