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Thread: Near Interpupillary Distance

  1. #1
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    Question Near Interpupillary Distance

    Hello people

    The matter is:

    The Near Interpupillary distance depends on the:

    1. object Distance ( work Distance )
    2. Lenses Dioptry
    3. Rotation Center
    4. Far Interpupillary Distance
    5. Vertex distance

    If I didnt forget anthing those stuff all can change the Near Interpupillary distance. The question is:

    HOW ABOUT THE PHYSIOLOGICAL FACTOR?

    Would anyone have another factor which could be considered?

    Thanks

    Edgley:hammer:

  2. #2
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    Say you have the near PD. What do you do with it? How do you intend to use it in dispensing PALs?
    If you really need it, How do you measure it?
    Raanan

  3. #3
    sub specie aeternitatis Pete Hanlin's Avatar
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    I'm not quite sure I have a total grasp of where you are going with all this (then again, I'm never quite sure I have a total grasp on life in general, so...).
    ;)
    Anyway, another factor to consider when computing NPD is the power of the distance portion of the lens (and more specifically, the prismatic effect it will have at the near viewing point of the lens). The stronger the minus power at distance, the more prism BI the patient will be looking through. The stronger the plus power at distance, the more prism BO the patient will be looking through.

    If you have lenses with relatively strong plus power, decentering the segment in farther will lower the total amount of horizontal prism the patient encounters at near (provided the segment is wide enough to allow this). Consider that looking through BO prism at near will require additional convergence by the wearer.

    Don't know if any of this is pertinent to your original question or not...
    Pete Hanlin, ABOM
    Vice President Professional Services
    Essilor of America

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

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    I attended a lecture (CEC type) where I was told that convergent P.D. could be as little as 2 mm on myopes and as great as 5 mm on hyeropes. Now the logical question: "Why don't PRD's have dial for the Rx to compensate for these things and accurately measure them? It is assumed that we are too dumb to calculate these things and therefore need a PRD to begin with, so why not have one that covers the bases?

    Chip

  5. #5
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    Edgley,

    Are you referring to the dynamics that the eye muscles may influence the near pupillary measurement?

    :)

  6. #6
    Master OptiBoarder Texas Ranger's Avatar
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    Smilie

    I think another factor which does affect pal wearers especially is physical size of the person,i.e. if the reading distance is desired to be 16" for each of two pts. pt #1 is 6'5", wears a size 8 hat, has a distance PD of 73mm; pt. #2 is a petite lady of 5'1", has a pd of 55mm, i do believe that the big guy's eye's must converge considerably more to see at 16" than miss 55mm; and the same goes for the range through the progressive corridor. think of it as a convergence box, squared, the big guy's box is about 20% bigger, very different adaptation problems.

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    Thumbs up

    Bev Heishman said:
    Edgley,

    Are you referring to the dynamics that the eye muscles may influence the near pupillary measurement?

    :)
    Yeah you guessed it right!!!! I was wondering if a muscles or another fector would change the way we measure it!!!

    thanks everybody

    Edgley

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    Raanan Bavli said:
    Say you have the near PD. What do you do with it? How do you intend to use it in dispensing PALs?
    If you really need it, How do you measure it?
    Raanan

    Ok, think about it. Let's supose you are in charge to desing a new PAL of +800D to the right eye and -800 to the left eye. So how would you set the progressives zones in both lenses ? Should it be diferent or not?

    See you

    Edgley

  9. #9
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    Edgely,

    I am in a very large practice and see many individuals who are not successful candidates for PAL's due to phoria's and high prism requirements.

    On the phoria's I find we can handle 1 to 1.5 in either BI or BO direction. I often will take a CRP measurement and then I observe the individual looking at near and dot the near PD. On this individual I specifically state the near PD not the distance.
    It works.

    We also recommend FT's and larger ones for our macular degeneration patients due to field problems. They help.

    I do have some unusual things. Recently there was a man with 9D BO OU combined with 3 UP OD and 3 D OS who claims his Varilux Comforts are fantastic? He also insists on a 56 eye frame and was very upset when the on lens did not surface all the way out. He came from another facility. By the way he was in 1.6 polished edges and he didn't want an AR.

  10. #10
    Master OptiBoarder Darryl Meister's Avatar
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    As both Chip and Pete pointed out, it is possible to compensate the near PD for the effects of prism.

    Near PD (or segment inset) can be adjusted to account for the effects of prism on convergence in order to ensure proper alignment of the bifocal segments. Since plus lenses produce base out prism at near, this increases the amount of convergence required to see an object at near, and vice versa for minus lenses. Consequently, in order to ensure that the centers of the bifocal segments are aligned properly in front of the lines of sight during near vision, the segments can be decentered by an additional amount for plus lenses. This moves the segments in farther from the normal near PD, placing them in front of the lines of sight, which have converged more because of the prismatic effects of the distance zone. For minus lenses, the lenses would actually be decentered less, or moved out from the normal near PD. This type of compensation is actually done in several modern progressive lens designs.

    The near PD (or segment inset) can be adjusted in order to neutralize the effects of prism on convergence. Essentially, this would leave the convergence equal to the amount that would normally be required by the person if he/she was wearing no lenses. This would require decentering the lenses even farther than the above compensation requires. This is generally not recommended since A) It reduces the effective (binocular) size of the segments and B) Wearers become accustomed to the amount of convergence required during near vision before they move into multifocals, so it really doesn't offer a meaningful advantage.

    As far as prescribed prism goes, you can ensure optimal performance by compensating the centration of the fitting cross for the amount of prism used. Remember that prism causes objects to appear displaced, which in turns causes a slight rotation of the eye in the direction of the displacement. The rule of thumb is to move the fitting cross 1 mm in the direction of the prism apex for every 3 diopters of prism. (Below 3 diopters, this compensation is probably inconsequential.) For instance, if you have 3 diopters of base in prism prescribed for each eye (6 total), you should move each fitting cross out by 1 mm or increase the total PD by 2 mm.

    Ultimately, the primary factors that affect the near PD (or inset) include the distance PD, the distance prescription, the vertex distance, and the working distance of the near object (which will be shorter for higher adds). Of course, other factors may also influence it, such as some binocular anomalies.

    Best regards,
    Darryl

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    Edgley:

    There is no way in H____ that anyone can wear a pair of spectacles with a 16 diopter difference in powers. The image size with any type forward lens would be far too different, the depth preception would be non-existant and the Ring Scotoma's in the +8.00 lenses would present a new set of problems. You might manage to have sucess with this in contact lenses, but even then you would probably have to reduce the + in the +8.00 to achieve image symetry at the cost of some distance acuity.

    This is why in the days of non-pseudophakic cataract surgery, one had to frost the lens on the eye with the poorest acuity until both eyes had been operated on.


    Trying to do this in a PAL would be beyond the futality of spiting into a strong wind.

    Chip

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    chip anderson said:
    Edgley:

    There is no way in H____ that anyone can wear a pair of spectacles with a 16 diopter difference in powers. The image size with any type forward lens would be far too different, the depth preception would be non-existant and the Ring Scotoma's in the +8.00 lenses would present a new set of problems. You might manage to have sucess with this in contact lenses, but even then you would probably have to reduce the + in the +8.00 to achieve image symetry at the cost of some distance acuity.

    This is why in the days of non-pseudophakic cataract surgery, one had to frost the lens on the eye with the poorest acuity until both eyes had been operated on.


    Trying to do this in a PAL would be beyond the futality of spiting into a strong wind.

    Chip
    Thanks Chip for your explanations. Although i didn't know that, the exemple that I gave was just to make Raanan Bavli think about the change that a lens can produce in a Near Interpupillary Distance. As I said before I was just wondering which factors would produce more diference in Near Interpupillary Distance.

    Thanks everybody again;)

    Edgley

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