Results 1 to 10 of 10

Thread: Multifocal 2.5mm 3.0mm descentration

  1. #1
    OptiBoardaholic
    Join Date
    Jun 2006
    Location
    Panama
    Occupation
    Optometrist
    Posts
    235

    Multifocal 2.5mm 3.0mm descentration

    I has been wondering how a fixed descentration of 2.5 or 3.0mm on multifocals is expected to fit a custommer with far P.D. of 72mm as well as another with 60mm far P.D. Each has a different convergence.

    Would this has something to do with the fact that once in a while a customer can not see thru the corridor at the same time with both eyes even if the lenses are correctly fitted?.

  2. #2
    Banned Jim Stone's Avatar
    Join Date
    Jan 2003
    Location
    Point Barrow
    Occupation
    Lens Manufacturer
    Posts
    340
    Quote Originally Posted by MIOPE
    I has been wondering how a fixed descentration of 2.5 or 3.0mm on multifocals is expected to fit a custommer with far P.D. of 72mm as well as another with 60mm far P.D. Each has a different convergence.

    Would this has something to do with the fact that once in a while a customer can not see thru the corridor at the same time with both eyes even if the lenses are correctly fitted?.
    What you talk about is not important. It falls under the "close enough" rule.

  3. #3
    Manuf. Lens Surface Treatments
    Join Date
    Aug 2002
    Location
    in Naples FL for the Winter months
    Occupation
    Other Optical Manufacturer or Vendor
    Posts
    23,240

    Blue Jumper wondering how a fixed descentration of 2.5 or 3.0mm

    Quote Originally Posted by MIOPE
    I has been wondering how a fixed descentration of 2.5 or 3.0mm on multifocals is expected to fit a custommer with far P.D. of 72mm as well as another with 60mm far P.D. Each has a different convergence.
    This fixed decentration is of the optical centers for distance and if they are set right it will place the near segments at the proper place for convergence.

  4. #4
    OptiBoardaholic
    Join Date
    Jun 2006
    Location
    Panama
    Occupation
    Optometrist
    Posts
    235

    Which method is better.

    Which method is better?
    Meassure the far P.D. or measure the near P.D. and add 2.5mm to calculate the monocular far P.D.
    I use the first (far P.D.) but saw a few months ago on the essilor site (in spanish) the second one was recommended.
    What i am more concern about is that the customer should be aligned with the corridor at every intermidiate distance he sees.
    I had recently a case (and has had some more) that everything seem to be ok but at certain distances the customer was seeing sharp thru one eye because the other was out of the corridor. I had to modify the far P.D. in this case increase it to solve the case.
    I am asking this questions to find a reason for this and try to anticipate the problem.

  5. #5
    Banned Jim Stone's Avatar
    Join Date
    Jan 2003
    Location
    Point Barrow
    Occupation
    Lens Manufacturer
    Posts
    340
    Quote Originally Posted by MIOPE
    I has been wondering how a fixed descentration of 2.5 or 3.0mm on multifocals is expected to fit a custommer with far P.D. of 72mm as well as another with 60mm far P.D. Each has a different convergence.

    Would this has something to do with the fact that once in a while a customer can not see thru the corridor at the same time with both eyes even if the lenses are correctly fitted?.
    I wear a pair. I'm pretty sure there are a few others having no problems.

  6. #6
    ATO Member HarryChiling's Avatar
    Join Date
    Apr 2005
    Location
    Nowhereville
    Occupation
    Other Eyecare-Related Field
    Posts
    7,765
    Quote Originally Posted by Jim Stone
    Quote:
    Originally Posted by MIOPE
    I has been wondering how a fixed descentration of 2.5 or 3.0mm on multifocals is expected to fit a custommer with far P.D. of 72mm as well as another with 60mm far P.D. Each has a different convergence.

    Would this has something to do with the fact that once in a while a customer can not see thru the corridor at the same time with both eyes even if the lenses are correctly fitted?.




    What you talk about is not important. It falls under the "close enough" rule.
    Actually the greater the distance PD and the higher the add power or Rx, the greater error this will introduce. I believe Zeiss has a lens available that has a variable inset, this will help with those that do require accuracy. The progressive makers use a standard average inset to avoid the cost of many different blanks. I would suggest getting in touch with Darryl Mesiter on this board and asking about the Zeiss lens, I would guess that this would help to solve your problems.
    1st* HTML5 Tracer Software
    1st Mac Compatible Tracer Software
    1st Linux Compatible Tracer Software

    *Dave at OptiVision has a web based tracer integration package that's awesome.

  7. #7
    OptiBoardaholic
    Join Date
    Jun 2006
    Location
    Panama
    Occupation
    Optometrist
    Posts
    235
    Harry:
    Thanks for this information. Is good to know that Zeis may have the solution. Now, i would like to have a method to know in advance if a person may need this special lens instead of the trial and error technic of ordering a regular multifocal and see what happends. That is what i do now.

    Since P.D. , Rx, Add, multifocal design, Vetrex distance and maybe a few more parameters added together can be the reason for this problem. I was wondering if there is more information about it.

    For example, let say i measure the far P.D. and th near P.D. to calculate convergence. How much convergence should be to suspect a problem?
    ¿Is there any multifocla manufacturer that has information about this?

  8. #8
    OptiBoardaholic
    Join Date
    May 2000
    Location
    B.C. Canada
    Occupation
    Dispensing Optician
    Posts
    1,189
    Convergences can be calculated by using similar triangles, thats why the inset used by manufacturers is generally very accurate, at least under normal circumstances. Where you often run into difficulties is in situations where there is a convergence disorder. or any alignment disorder. In some cases one eye may track the corridor ,while the other eye may "wander" and actually miss both the corridor and the reading segment. If the deviation is intermittent, and it often is, you may find your near pd measurements to be of questional value. In rare instances, we have swung the progressive zone outwards in one eye, and added 10 degrees to the axis to allow for it. I have posted before about the habit some people have of covering one eye up, then the other, then complaining they have to make small head movements to see clearly monocularly. Since there is no stimulus to converge, there may be a slight deviation of one eye, or both, and the "test" has no merit. I just ask if they can read ok with both eyes open, and when they say yes, I tell them their glasses designed to be used with BOTH eyes open. Hope this helps a bit.

  9. #9
    OptiBoardaholic
    Join Date
    Jun 2006
    Location
    Panama
    Occupation
    Optometrist
    Posts
    235
    Quote Originally Posted by Dave Nelson
    I have posted before about the habit some people have of covering one eye up, then the other, then complaining they have to make small head movements to see clearly monocularly. .
    I admit i has used this method to try to figure out how off is one eye from the corridor but has not think about the lack of stimulus to converge that you mentioned. You are right. This is not a good method.

    I keep looking for a method to detect in advance those cases where the corridor is off on one eye but the far P.D. is ok on the lens. Due to Lack of convergence, etc.

    Would it help if we measure the far and near monocular P.D. of each customer that is ordering Multifocals?. With a pupilometer this takes less than a minute. But, how far from 2.5 mm of convergence on each eye should be enough to suspect the need of a compensation?

  10. #10
    OptiBoard Novice
    Join Date
    Nov 2005
    Location
    london
    Posts
    6

    Wave zeiss lens

    hello,
    u need to dispense zeiss individual. they will give you a correct inset indeed and will be tailored made.
    the measurements needed are, B.V.D, pantascopic tilt, distance monocular pd, near centration distance or near mono pd at px working distance, bow of the frame, fitting cross position and prisim thinnig needed other wise 2/3rds of add. and px will get the widest intermediate corridor with widest reading area.

    please also mention if px does more eye moments or head moments when looking at side objects.

    problem with wider pd is that nasal unwanted astigmatism increases and there will be a problem with int corridor. in this lens they will work out the exact distance from Distance visual point to near visual point and will give the smooth progression and without the doubt it will be the most comfortable pair px would hv had.

    regards

    Amit jain.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Descentration calculator
    By gola in forum General Optics and Eyecare Discussion Forum
    Replies: 1
    Last Post: 02-27-2006, 12:56 AM
  2. MultiFocal Contact Lenses
    By Catie in forum General Optics and Eyecare Discussion Forum
    Replies: 14
    Last Post: 08-25-2004, 06:42 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •