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Thread: pantoscopic tilt and face wrap

  1. #26
    OptiBoardaholic
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    I do not think that the first prescription
    -6.62 -2.95 × 180
    -7.13 -3.22 × 180
    is compensated twice but can not say that I am 100% sure.

    This is how I see this.
    Most of the time you input the Drs. RX also known as uncompensated Rx and frame pano and frame panto and a compendated Rx is calculated. When the lab grind the lenses they expect to read in the lensmeter the compensated Rx.

    In this case we go in the other way.
    I have a frame with two lenses that I measure to get the compensated Rx and have a frame from where I get pano and panto. This is the information of the glasses the patient is wearing.
    With this information (like reverse ingeneering) I calculated a uncompensated Rx. And use it as input to calculate a new pare of glasses.

    If this is true, the prescription
    OD -6.50 -2.75 x 180
    OI -7.00 -3.00 x 180
    Is the compensated one.

    But might be wrong and this is why I posted it to hear other opinions.

    And why I will do two jobs on the lab. One with each prescription.

  2. #27
    Master OptiBoarder OptiBoard Silver Supporter lensmanmd's Avatar
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    Forgive me if my consumption of Speyside affects my response. I just returned from a pretty awesome party.

    I would request that you trust the IOT FF calculator to provide the correct compensation values. I do believe that you are over thinking this.

  3. #28
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    The Rx you are giving that is "compensated" just doesn't look like a "compensated" Rx. I don't think I've ever seen one where it outputs to the quarter diopter like that. Compensated Rx's typically look far different. I get the feeling the Rx you had was the uncompensated Rx, and the Panto and Pano given were what needed to be input with that Rx for the original glasses.

    Maybe I'm wrong, but it just seems off.

  4. #29
    Master OptiBoarder lensgrinder's Avatar
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    Quote Originally Posted by drk View Post
    B-man:

    What's the explanation of what "effective power" is?

    I think I know what "compensated" is...you put in the "flat" Rx and get out the lens powers in a curved frame that will provide the "flat Rx".
    Quote Originally Posted by lensmanmd View Post
    If I remember correctly, compensated power insures that the effective power equals prescribed power for any given frame/lens combo.

    Lensgrinder could probably provide the geek-ed out version of this.....

    It's late and the vodka is kicking in. Too much math for me.
    I love to geek out, but it seems I am late to responding to this thread.
    DRK, the effective power is the effect of changing parameters. For example, when you prescribe an Rx of -10.00 at a certain refracted vertex and the glasses have a different vertex distance the power have an effective change in power, meaning the power reads -10.00 in the lensmeter, but the effect of the patient is different. So we compensate for the effective change.

    Quote Originally Posted by MIOPE View Post
    Thanks Lensgrinder.
    The effective power on this calculator seems to be what I need.
    But do not know why sometimes this calculator gives axis 61.18 as result when the input Rx axis is 180.
    I will need to look at these calculations, they seem to be correct on my app and on the python script. 61 degrees is too much of a change.
    Please do not use these calculations for this purpose, as others have stated. These calculation from Keating are not meant for lenses with thickness and curvature. The manufacturers will ray trace the lens in the intended position to determine the necessary compensation. I understand what you are trying to do, but I would not duplicate a compensated Rx.

  5. #30
    sub specie aeternitatis Pete Hanlin's Avatar
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    Super late to the thread, but...
    I would request that you trust the IOT FF calculator to provide the correct compensation values. I do believe that you are over thinking this.
    +1
    As you stated earlier, the patient does not always prefer the final Rx you land upon in the phoropter. However, assuming you've checked binocular vision to ensure you've achieved "20/Happy" and you've trial framed the Rx at distance and (if applicable) near, the subjective portion of the Rx process is complete and you've done all you can to provide the best vision to the patient from a refraction standpoint.

    But physics is physics, and will always be.

    +1 again
    Once you have identified the Rx the patient has subjectively indicated provides the best perceived vision, it becomes a matter of physics. At this point, the only variable is lens geometry and positioning. The assessment in the phoropter and trial frame have been performed with the visual axis traveling perpendicularly through the optical center of the lens, which is very rarely the case in a finished pair of eyewear (and is never the case for a progressive lens).

    At this point, there are actually two formulas that come into play (plus a third consideration that is beyond my ability to calculate). Martin's Formula for Tilt will determine the amount of cylinder created by whatever tilt is present in the finished eyewear, and Thompson's Formula for Obliquely Crossed Cylinders will determine how that cylinder will affect the axis and power of any cylinder present in the Rx. Assuming you were dealing with spherical optics (which is never the case with progressives, and almost never the case with digitally surfaced SV lenses), you could probably figure out the effects the lens position will have on the lens and perhaps reverse engineer an Rx to be ground in the lens which would provide the patient with the prescribed effect in the as worn position. It is sometimes surprising how much the axis of a cylinder is effected by compensation, but its pretty common to have a large "change" with a lens that has a large sphere power but a low cylinder power (because the cylinder identified by Martin's can have a large influence on the axis of a low powered cylinder when Thompson's is applied). Now the third calculation will concern any asphericity present in the front or ocular curve (because- as lensgrinder noted- you also have to account for lens shape). This is where you'll require a bunch of information which is not usually available (e.g., what is the p-value of the aspheric shape). Applying Martin's and Thompson's formulas to aspheric curvatures becomes a- oh, I just got a migraine thinking about it.

    Fortunately, if you just provide the results of the subjective refraction, the IOT calculator (or whatever manufacturer's calculator you are using) will crunch through all this information for you (because the calculator will have all the data regarding the asphericity of the lens and what not) and inform the generator accordingly (along with a compensated Rx for you to use when confirming the power of the lenses).

    In other words- "What lensmanmd said!" :)

    BTW, I love your definition of retirement- and I'd like to think I will spend at least part of mine in an optical laboratory (after all, that's where I started :^)! However, I would really advise against trying to reverse calculate compensation.
    Pete Hanlin, ABOM
    Vice President Professional Services
    Essilor of America

    http://linkedin.com/in/pete-hanlin-72a3a74

  6. #31
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    I want to thank to everyone that has post their opinion. I have learned a lot.

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