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Thread: OC Heights

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    ATO Member OPTIDONN's Avatar
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    OC Heights

    How many of you take optical center heights? Should they always be taken on aspherics? I know to minimize power error the lenses axis should pass through the eyes center of rotation but if an optical center height is not taken how could the axis pass through the center of rotation?

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    Bad address email on file QDO1's Avatar
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    Quote Originally Posted by OPTIDONN
    How many of you take optical center heights? Should they always be taken on aspherics? I know to minimize power error the lenses axis should pass through the eyes center of rotation but if an optical center height is not taken how could the axis pass through the center of rotation?
    Always, on everything

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    Master OptiBoarder Darryl Meister's Avatar
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    Oddly enough, I actually overlooked this thread.

    While it's certainly not a bad idea to take a pupil height measurement, it's usually not a great ideal to vertically decenter the optical center to the pupil height. Yes, you disrupt the relationship between the optical axis of the lens and the center of rotation of the eye when this is done, but you also increase the maximum thickness of the lens blank. This is a particularly important consideration for high-index and aspheric lenses, since the wearer is actually paying for a thinner, lighter lens.

    You may consider raising the optical centers some when fitting the wearer with a low-Abbe material in a high power, in order to minimize chromatic aberration, but even then I wouldn't suggest raising the optical centers all the way to the pupil height. Consequently, if you do take pupil height measurements, you don't necessarily want to specify those heights without some consideration.
    Darryl J. Meister, ABOM

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    What Darryl said is true from an asthetics point of view. But If you don't take the vertical height into concidersation, especially on low abbe materials. You can end up with a lot of patient's telling you about those yellow and red outlines around everything. Unfortunately as styles are appearently about to dicatate larger lenses again, you can look for more trouble in this reguard. Unfortunately higher cylinders and higher powers will also accentuate the problem.

    Of course this is why poly is a junk material for high powers.

    Chip (I hate to disagree with Darryl twice in a week).

  5. #5
    Master OptiBoarder Darryl Meister's Avatar
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    Quote Originally Posted by Chip
    I hate to disagree with Darryl twice in a week
    I don't know:

    Chip: "If you don't take the vertical height into concidersation, especially on low abbe materials. You can end up with a lot of patient's telling you about those yellow and red outlines around everything,"

    Sounds an awful like:

    Darryl: "You may consider raising the optical centers some when fitting the wearer with a low-Abbe material in a high power, in order to minimize chromatic aberration."

    ;)
    Darryl J. Meister, ABOM

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    Quote Originally Posted by Darryl Meister
    Oddly enough, I actually overlooked this thread.

    While it's certainly not a bad idea to take a pupil height measurement, it's usually not a great ideal to vertically decenter the optical center to the pupil height. Yes, you disrupt the relationship between the optical axis of the lens and the center of rotation of the eye when this is done, but you also increase the maximum thickness of the lens blank. This is a particularly important consideration for high-index and aspheric lenses, since the wearer is actually paying for a thinner, lighter lens.

    You may consider raising the optical centers some when fitting the wearer with a low-Abbe material in a high power, in order to minimize chromatic aberration, but even then I wouldn't suggest raising the optical centers all the way to the pupil height. Consequently, if you do take pupil height measurements, you don't necessarily want to specify those heights without some consideration.

    I think this is a balanced approach which deserves a lot of merit. I sometimes wear poly (approx -3.50) and I don't have to look up and down to see chromatic aberration. In fact, I see it mostly when looking side to side. The point is my lenses are properly decentered for my PD, but not for my pupil height. So, as Daryl says, don't necessarily decenter a lens upwards "all the way" to the pupil center if it will significantly thicken the lens.

    We use this approach on PDs sometimes too. If a patient wears > +/- 2.00D, and their PD is 60, and their habitual Rx has an MPD of 71, it would probably be a mistake to make new glasses with a 60 PD.

    Heck, you can throw a patient off who was wearing a large B frame, where their pupil was a mile above the OC, just by putting them into a more narrow (sensible) frame. Had one the other day. A gentleman had reverted back to his old, thick lenses (+7) in a large B frame, because "he could see better" The new ones were the same Rx, not that narrow, same material, very close base, hands down a better fit. Go figure.

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    Bad address email on file QDO1's Avatar
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    I measure heights (and a whole load of other measurements) on everything because

    • when it absoloutley does matter, I will be uterly fluent with taking the measurements
    • it eliminates a lot of potential problems caused by prism, power and ABBE etc
    You would be daft not to

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    ATO Member HarryChiling's Avatar
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    I would only move the OC's in anisometropic patients, otherwise you end up with prism imbalance and in some cases (not very many) the patient will complain of double vision, however; even when I move the OC I do take into consideration the aesthetics of the lens and sometimes may split the difference in vertical decentration.
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    When taking OC heights I have found it important to check their old eyewear an find out where the OC are fitting if you change OC in high + or - the patient may not tolerate

  10. #10
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    I always check for vertical and horizontal placement in aspherics. My optimum to have them work and have the best cosmetics is to minimize the decentration and vertical differences to + 3 or minus millimeters in either direction. I also use this philospophy in any mid to hi index material. I have never had a remake problem. I also pay attention to the vertex distance and wrap.

  11. #11
    ATO Member HarryChiling's Avatar
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    Bev, I dont know where that came from but it is a general rule of thumb I teach others to keep the decentration to 3mm preferably in, if it is out mention to the patient a larger size would be better but that the frame still fits.
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    Professional Rabble-Rouser hipoptical's Avatar
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    I know I'm relatively new to Optiboard, and this is my first time to delve into this forum, but in case you want my two cents...
    It has been my practice for years to not allow a frame fit that would decenter any lens more than 5mm. Less than 5 is preferable. We try to ensure that vertical alignment is centered as well, so we do not take OCs unless there is a greater than 3mm rise needed. We also dispense A/R coated products in the greater majority of our adults (96%) which alleviates the effects of low abbe lenses, as well as other problems. (I don't know why any of us even offer lenses without A/R anymore, but that's another discussion...)

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